Edwin_m

The below is something of an oversimplification but I think explains in general terms...

Transformers have been around for many years so AC could be transmitted at high voltage through the overhead (greatly reducing losses) then stepped down on board to a voltage suitable for the motors.  When these systems first appeared there was no good way of converting AC to DC on board the train, and AC motors aren't very good at running at a variable speed unless fed with a variable frequency, which wasn't possible until many decades later. 

However, if I remember this rightly, a brushed motor while essentially a DC machine will work on low-frequency AC.  When the supply voltage changes polarity, the current reverses in both the armature and the field coils, so the forces between them continue to turn the armature in the same direction.  (Separately the brushes change the polarity of the armature only as it passes each field coil, so that the torque continues to pull the armature towards the next coil).  But because the coils in the motor are inductors, their impedance increases at high frequency so a greater proportion of the energy goes towards heat in the coils (the same reason DCC users are warned about using a DC loco on address zero).  Clearly this was too much at industrial frequencies but acceptable at 16.67Hz, which could be produced by a trackside rotary converter (motor-alternator set) with a 1:3 ratio between its two sets of coils.  

9 hours ago, jpendle said:

They preserved the flying arches at Chorley so I don't see any reason why a listed footbridge shouldn't be given the same treatment.

 

Regards,

 

John P

The flying arches are listed and also unique.  There are several surviving NER footbridges.  If this one was kept it would have to be raised on a plinth, high enough to maximise the distance between the wire and the platforms, which would completely spoil the looks.  Best option would be to donate it to some deserving preserved railway.  

56 minutes ago, melmerby said:

It also says that although the sites at OOC and Euston have been cleared no construction has taken place.

!s that correct?

Some work beyond site clearance is in progress at both sites, although neither has much that is recognisable as a station.  For example, at OOC the box is being excavated and at Euston new Underground ventilation passages are being dug to allow demolition of the former station on the west side where the ventilation now is or was.  

Don't put it in the wheelie bin or it will probably melt it!  

1 hour ago, doilum said:

This was the reasoning behind medieval retailing with all the baker's on Baker street and fish mongers on Gillygate. Customers could make direct comparisons and the shopkeepers found that having their rivals close by had a "gravity" effect drawing potential customers from a wider area. No doubt the arrival of the internet has caused all this economic geography theory to be rewritten, but the advent of independent food delivery firms may have created a new benefit from clustering together.

It's still considered to be significant by the people who claim to understand such things.  It's actually cited as a reason to bring the cities of the North closer together by building a new railway between them to reduce travel times, so that they have the same sort of "critical mass" as London.  Not that that means anyone will be going from Manchester to Leeds to get a bucket of greasy chicken.  

An example of "agglomeration effect"*, rather like people might go to Hatton Garden to shop for jewellery?  If people know they go to a certain place for piri-piri, then find their preferred outlet is closed or has a queue, or the other one just looks nicer, then maybe they'll try that one instead?

*This may or may not be a comment on their sauce.  

13 minutes ago, Reorte said:

 

Whilst catch points are certainly much less frequently needed sprung points I'm pretty much sure are still a thing. Someone correct me if I'm wrong (because I quite possibly am) but I believe that the passing loops on the West Highland line are all spring points, I believe it's why they go to the opposite platform to expected (the right hand side) at Corrour, so that the siding can be used with less hassle from the sprung points.

They're certainly self-acting, though I think the mechanism is more complicated than a simple spring.  I seem to recall the original version had to be replaced because parts were no longer available.  

Some areas with bi-directional signalling are fitted with "Patrollers Lock-Out Devices" or PLODs, which inhibit the setting of any "wrong direction" route while someone is patrolling the line.  Patrollers normally have (or had, largely extinct these days) to walk in the four-foot to be able to carry out their inspections.  

5 hours ago, The Johnster said:

 

????

 

The 4-foot, the space between the rails, shorthand for 4'8.5", is without doubt the most dangerous place to be on the railway apart from actually on or across one or both of the rails.  The 6-foot, the space between parallel tracks, is not exactly recommended either; the advice given me on my guard's training course was that, if I should ever be caught there between passing trains (never was, btw), the best thing to do was to lie down flat on my stomach and put my hands over my head.  But if I had to choose, I'd lie down in the 6' any day over being mangled by the AWS shoe at 90mph, or even 15mph, in the 4-foot!

That's not what I was taught on track safety courses from the late 1980s until mid-1990s and as far as I'm aware it's the same today.  

It surprised me too at the time, but the logic is that from the four-foot it's relatively easy to get to the cess, a place of relative safety with only the one rail to cross.  From the six-foot you have to cross two rails, and if two trains approach simultaneously you don't know which way to go.  Lying down in the six-foot was indeed the recommended advice if trapped in that situation.  If you're on the track when a train approaches then the four-foot is no doubt more dangerous than the six-foot, but the six-foot is the most dangerous place to do any on-track work.  

Just done an image search for voidmeter, and none of them look like the one in the OP.  

9 hours ago, Wickham Green too said:

.... but why in the 4' rather than outside the rail where it's a little safer to work  ? - and why no sign of anybody working it ? ....... there must have ben some reason for photographing it - unless this is only a tiny out-take from a bigger scene.

Perhaps because the four-foot is safer than the six-foot?  

It appears to coincide with a fishplate, ballast has possibly been added to the sleeper bays each side but the one in question appears to have less ballast.  I wonder if it's some kind of jack to support a dipped joint.  

Trap points have to be restored to their normal (trapping) position at all times except when a legitimate move is signalled.  On mechanical signalling I assume this is just done by instruction, but in power signal installations they either have a flashing reminder on the panel, or they auto-normalise unless specifically keyed into the reverse position.   

Even if wagons are secured with handbrakes, it's been known for them not to be sufficiently applied or for vandals to release them.  In those situations the trap point is the last line of defence.  

10 hours ago, Northmoor said:

It's a bit of a misconception that HS2 is expensive because it is built for 225/250mph.  A new line built for 125mph would be as straight as possible; Pendolinos only have tilt as a bodge to compensate for the fact they have to operate on a curvaceous Victorian railway.  No-one would build a new high speed railway that actually needs tilting trains to achieve a high operating speed.

 

Oh and an aside on capacity.  I commute into London (normally) 2days/week and it's very obvious that the trains are busier week on week.  Even tonight, my 12-car from Waterloo (which was pre-pandemic busy at 1730) had 4 people in each six-seat bay towards the front, while the guard was asking people to take bags off seats further back on the train where there were lots of standees.  So as I predicted over two years ago, commuter traffic is definitely returning and the stats show that leisure travel now exceeds pre-pandemic levels (speaking of which, I have never seen traffic levels on the M25 at any time, like last Sunday).  

Indeed.  If it was built for 125mph you might reduce the track spacing and clearances a bit, and simplify the overhead line and power supply a bit, neither of which is very significant in relation to the total project cost.  The main difference may be tunnel cross-sections, but several of the longer tunnels won't be cleared for maximum speed anyway.  

The London-Birmingham route is largely able to achieve a good alignment, although gradient restricts speed through the Chilterns.  Further north, mainly on sections now dropped from the scheme, there were parts of the route where lower speeds were accepted to curve round obstacles, rather than introduce extra tunneling under settlement or through hills.   

A short between the pickups and the motor connections (perhaps better described as an unwanted connection, as it doesn't actually cause a short circuit) wouldn't affect operation on DC - it's just another way of getting the current from the pickups to the motor.  But when a decoder is used, that short will be applying track power to the decoder components that are intended to drive the motor, and this will almost certainly destroy them.  

4 hours ago, jjb1970 said:

 

My understanding was most of the modern locomotive platforms are modular with common traction packages and control & electrical systems and that the diesel versions can be re-engineered to electrical operation mid-life. 

Many post-privatisation EMUs have provision to be converted between 750V third rail and 25kV overhead, but unless I've really missed something no British DMU design is easily convertable to electric.  Even the Voyagers, which have electric transmission, were found to be difficult when it was proposed a few years back to add an extra coach with a pantograph and transformer.  There would have been problems with the new cables feeding the power through the train to the motors.  

6 hours ago, jjb1970 said:

It seems to be increasingly the case that diesel trains are being designed to be convertible to electric power supply later. Makes a lot of sense, given the time frames to convert lines to electric power and the typical life of a locomotive or train in many countries.

It definitely should be, but I'm not convinced it actually is - certainly in the UK?  Can you offer any examples?  

The CAF class 331 EMU and 195 DMU are visually almost identical, but the 195 uses mechanical transmission so isn't easily convertible to a 331 - probably no provision for a pantograph or transformer either.  

On 25/10/2022 at 15:04, jjb1970 said:

Mechanical transmission is very efficient, provided the engine and gear ratios are matched to the operating profile (a significant if). Manufacturing gearboxes is expensive, especially for high power applications. Hydraulic transmission is compact and lightweight but that's not the same as being efficient in terms of transmission losses. Electrical drive is very robust and flexible (it's hugely beneficial to equipment layout if you don't have to worry about a shaft line or hydraulic connections with the associated issues) and the torque characteristic of electric motors is ideal for transport application. On the other hand it's heavy and can be expensive. So really there's no such thing as a perfect transmission technology, they all have a place.

With electrification plans being scaled back, we are likely to see more DMUs ordered.  These are likely to have hybrid drive, convertability to future electric/battery/hydrogen, or the maximum commonality of components with EMUs.  This most likely points to electric transmission, helped by the availability of mass-market components from the automotive market (just as happened with mechanical transmission for the first-generation DMUs).  

5 hours ago, KingEdwardII said:

That all depends on the journey involved. Trains are fine if the journey is city-to-city along an established route. But there are many journeys where the train rapidly becomes a pain and the car is easier.

 

Our own journey from Hampshire to Northumberland to visit family is an example. We can get a train from Winchester to Alnmouth - either the faster route via London with the hassle of the Tube or a taxi, or the slower route via Birmingham New Street. However, this leaves us with the problem of getting from home to Winchester and from Alnmouth to our relations' place. Expensive taxis or having to rely on our relatives to pick us up. We have always driven there as a result. 

 

We have used Alnmouth and the train once in Northumberland - to travel to Newcastle and to Edinburgh, but used our own car to get to Alnmouth station from where we were staying.

 

Yours, Mike.

That illustrates how HS2 will only be successful if stations are "fed" by other public transport links, so as many people as possible can leave the car at home or not need one in the first place.  By serving city centres near the existing stations, it can take advantage of the existing feeder services (not that they are always very good!), but public transport in rural areas is always going to be much more sparse.  That's why HS2 also provides park-and-ride near Birmingham and Manchester, so someone can park there and get to the centre of another city where a lot of people have their destinations.  But most journeys from one rural location to another one are probably always going to be easier by car, because someone trying to use public transport will probably need a car to reach their final destination.  

4 hours ago, Ron Ron Ron said:

 

It is quite remarkable.

This method has been used elsewhere on not only this project, but others.

I'm not sure if the Canary Wharf Crossrail station was built using a similar method?  I can't remember offhand.

 

I'm intrigued how they can stop thousands, if not hundreds of thousands, of tons of concrete structure from collapsing into the excavations and keep the whole thing stable, until the thing is completed and becomes a totally integrated, load bearing and supporting underground structure.

Maybe the man with the stick,in that photo above,  is holding it all up?  🤔.

Looking at the photos, I think they are building some hefty cross-members as they go, though I may not be fully understanding what is being shown.  Typically this sort of structure will have these, which hold the sides apart during construction and afterwards, and can also support the floor(s) above track level.  

There is another form of "top down" construction that still involves building the bottom first, though probably not used much these days.  Thinks of the caissons used to build things like the piers for the big Victorian railway bridges, where more height was added to the top and the combination of the extra weight and workers digging out at the bottom caused it to sink into the ground.  

14 minutes ago, Hobby said:

 

So he's got a split personalty then! ;)

Wouldn't he then be called Banana?  

Did it have to shunt to make way for a faster train?

3 hours ago, The Johnster said:

Mention of a 3-road tunnel has made me realise that I cannot think of any prototype example of such a thing anywhere, and would now be interested to hear of any for no other reason than my own inquisitivness!  I am not an expert in tunnelling technology or engineering, but I do know that it is expensive, difficult, and dangerous, so I would assume that 2-road tunnels were/are considered the norm, as 2-lane road tunnels seem to be, and that if more roads are required it is cheaper, easier, and less dangerous to build extra tunnels.  

I think the reason is that the arch of the tunnel has to be circular (or nearly so) to hold up the earth above it, and usually the curve continues down to trackbed level.  One or two tracks fit quite well into a bit more than a semi-circle, but with three or more the crown of the tunnel would be a long way above the top of the trains, so a lot of earth would be dug out unnecessarily.  Therefore, as you say, anything more than two tracks will generally be a combination of single and double track bores.  

There are a few exceptions, but I would hazard a guess that these are cut and cover tunnels, where the earth above the track has to be dug out anyway as part of the construction.  The Marsh Lane tunnel posted above looks like an example of this.  

19 hours ago, Michael Hodgson said:

I can't help wondering whether a track layout serving the old York Road Platform could have been compatible with the relatively recent Platform Zero.  The works associated with the reinstatement of the disused Gaswork Tunnel and the station remodelling would certainly have been different.  Kings Cross already has a platform 9¾,  they could have called this one number minus one!

Looking at the space available, I think a connection to York Road for a decent length of train would have made it impossible to use the easternmost of the tracks through Gasworks Tunnel for anything else.  In any case, the connection for return services on the other side of the station has now disappeared underneath the newish station building extension.  These curves were always heavily restricted due to clearance, so probably couldn't have taken even the few classes that are cleared for the Thameslink core.  

There would also have been the flexibility for a train from London to depart from Derby in either direction.  Without having any particular knowledge of the subject, I'd guess that the milk tanks would have been better on the rear, so they could be taken off by another engine at St Pancras and moved quickly to wherever they were unloaded.