Short overlaps - signalling question.

[OldNick]

I am trying to work out what signalling will be needed when an extension board is inserted into our club layout, and in particular when overlaps are insufficient. 

 

Period / setting:  1950s semaphore signalling on an ex MR secondary double line.

 

Situation. In succession, the down line encounters the following on its approach to a station.

 

1: a tunnel, emerging into a curved cutting with limited visibility (you can tell it's a model!), curving left 

2: a trailing crossover to the up line. 

3: a facing junction splitting the line into a fast and slow line 

4: on the slow line, a trailing junction into a headshunt, leading into an MPD.

5: both lines leading into platforms.

 

Being a model, everything is too close together, in particular 2, 3, & 4, with a reasonable (although only a scale 200 yard) distance from 4 to the start of the platforms.

 

The trailing crossover 4) is for loco access only to the MPD, with wagon access provided from an extended goods yard siding. 

 

The question is what signalling would be provided on this down line?

We're assuming that the tunnel length is long enough that some signalling would have to be provided between the tunnel, and 2/3/4, but how would the signalling for this collection of obstacles? 

 

[The Stationmaster]

Interesting one this.  At that time the standard overlaps were 200 yards for 4 aspect signals and 300 yards for 3 aspect although shorter overlaps could be used with either restrictions of line speed or special controls.  However another important criteria was the distance between running signals and facing points and while some of this was a hangover from semaphore practice - with many signals fairly close to the points through which they read I'm fairly sure the maximum was not much different from the length of an overlap (Signal Engineer will probably have the exact distance to hand I expect). 

 

Anyway in your case - with all the usual compression which results from a model railway I think the answer could be relatively simple and the trailing crossover would not be inside the overlap distance, so the protecting signal won't be on the scenic modelled area.

[TheSignalEngineer]

Can you post a sketch of the layout with the distances? Also how long is the tunnel?

[Edwin_m]

OP mentions following semaphore practice - so surely we are just talking 440 yards clearing point after the first stop signal? 

 

If the tunnel is about this length, would a stop signal have been located at the far end?  If so no question of overlap arises in the visible area. 

[The Stationmaster]

OP mentions following semaphore practice - so surely we are just talking 440 yards clearing point after the first stop signal? 

 

If the tunnel is about this length, would a stop signal have been located at the far end?  If so no question of overlap arises in the visible area. 

Good point Edwin - I just read 'overlaps' and made the obvious connection!

 

If it is semaphore then we're in a very different ball game as there would possibly be an acceptance Home Signal back in rear of the tunnel to provide the 440 yard Clearing point if the track layout was going to be busy.  The positioning of other signals would then be very much influenced by the track layout.

[bécasse]

With semaphore signalling as you specify, normal practice would be to provide a splitting signal just before the facing point and its locking bar, the reason being that this would prevent the fpl bolt being pulled and the point reversed after a train had passed the protecting signal but before the train reached the point.

 

There were ways round this though if it was inconvenient - you could provide a running shunt dolly at the point (a driver being expected to stop if a facing dolly was "on" even if the running signal had been off) or, of course, there could be a locking track circuit between the splitting signal and the point itself (in which case there would be no locking bar).

[Grovenor]

In this scenario you would really want the splitting signal to protect the trailing crossover, so given 1950s assuming a track circuit to lock the fpl would suit. The other signals probably required would be ground shunts to drop back into the loco sidings, and to come out again. Shunts could also be provided for moves over the trailing crossover, unless its right outside the box and has infrequent use which could then be handsignalled.

The physical configuration of your splitting home will need careful thought to get the best sighting from the tunnel, round the bend. Might be wise to assume banner repeaters at the other end of the tunnel.

Keith

[TheSignalEngineer]

If the distance between the trailing crossover and the facing points is less than about 60', then it would not be usual to provide an FPL Bar. It is possible to put one along the insides of switch rails but an absolute bu$$er in practice. The points in this case would be track circuit locked. That raises another issue in "Time of Operation Locking". This is to cover the eventuality of the train passing the Home signal at danger or the Signalman replacing the Home signal and starting to pull the FPL lever a fraction before the train operates the track circuit thus freeing the points to be moved. On the LMS this was covered by the track circuit extending for at least 120' from the toe of the points on the basis that the Signalman would either realise what was happening and re-bolt the points or would have time to fully move the points to the other position.

[OldNick]

Gents,

 

Many thanks for the replies so far, which I first got a chance to read this evening - sadly after I left the club for the evening so didn't get a chance to measure distances.

I'll knock up and post a sketch tomorrow with estimated distances

A couple of quick points - the tunnel length is undefined, but it goes under a substantial hill so would not be short.

We run 7-8 Carr trains, + loco as routine, and equivalent or longer mineral trains - the Midland Pullman looks like a short train when it is diverted onto this route - so over compression will not look sensible.

Plan tomorrow!

[OldNick]

 

Here's a sketch plan of the area - with a couple of other details (bridges) added.

 

Rough distances - scale feet.

 

Tunnel mouth to crossover 300'

Crossover to facing junction 150'

Facing junction is partially under a minimum vertical clearance road bridge 

Facing junction to trailing crossover MPD entrance 200'

Trailing crossover MPD entrance to bridge over canal 300'

Bridge over canal to platform end 50'

Platform length 500'

Down starting end of platform to signal box 100' 

 

[TheSignalEngineer]

Will have a look later when I have finished kitchen decorating duties, but first impression is signal box will be too far from points for mechanical working. In that period there would have been a box somewhere to the left hand end by the road bridge.

[OldNick]

I'd been wondering about that - definitely too far from the box for mechanical working, so had been wondering about electric motors, or a second box. Meacham South sounds like another urgent task then.

 

I'm guessing that it would probably have lasted until the Line was converted to colour lights, or closed.

[TheSignalEngineer]

A box to the left of the bridge on the Up side would be my choice, accessed either by steps from the bridge or a cinder path along the cess from the station. The latter would depend on the design of the bridge over the canal.

 

You say the road bridge is minimum vertical clearance, but the position of the points would imply it has to be fairly wide. There were some low clearance boxes built by the Midland to give a view under a bridge such as the one at St Andrews Junction in Birmingham, just visible under Garrison Lane in this picture.  http://www.warwickshirerailways.com/lms/mrbhm_sa1900.htm

 

As you would want some flexibility in the use of the crossover I would assume a Down Outer Home signal on the other side of the tunnel and an Up Starting signal between the crossover and the tunnel. The proximity of the station and junctions with two boxes gives the opportunity of some interesting close section working. A bit more thought needed there.

 

What is the layout at the right hand end, and how is the MPD accessed? is it only right line via the crossover or can it be wrong line from the station?

[OldNick]

 

You say the road bridge is minimum vertical clearance, but the position of the points would imply it has to be fairly wide. There were some low clearance boxes built by the Midland to give a view under a bridge such as the one at St Andrews Junction in Birmingham, just visible under Garrison Lane in this picture.  http://www.warwickshirerailways.com/lms/mrbhm_sa1900.htm

 

As you would want some flexibility in the use of the crossover I would assume a Down Outer Home signal on the other side of the tunnel and an Up Starting signal between the crossover and the tunnel. The proximity of the station and junctions with two boxes gives the opportunity of some interesting close section working. A bit more thought needed there.

 

What is the layout at the right hand end, and how is the MPD accessed? is it only right line via the crossover or can it be wrong line from the station?

 

The idea of a low-clearance box is interesting - I'd been wondering about a high box, but this seems more suitable, especially as, in modelling terms. it's right at the front edge of the layout. . 

 

I'd certainly been intending to have the up advance starting signal between the crossover and the tunnel.

 

As to the right hand end, there is a goods yard accessed by trailing from from the down loop, before it rejoins the down main. The main station box is at this point. There is then another trailing crossover between the main lines. Coal / ash access to the MPD is from this yard, but the expectation was that loco access would be via the access crossover drawn, with either wrong line light engine workings,  or by pulling forward to the down end crossover, and right line running up the up line, and then back down using the up and crossover drawn.

[TheSignalEngineer]

Unfortunately i haven't been able to find a picture of St Andrews Jn other than the link I posted, but Thurgarton was a similar design, albeit with the stairs dog legged to fit the site. http://tillyweb.biz/pmc/thurgarton061975pmc1.jpg

 

There are also several on this page http://www.derby-signalling.org.uk/Newark.htm

[OldNick]

Unfortunately i haven't been able to find a picture of St Andrews Jn other than the link I posted, but Thurgarton was a similar design, albeit with the stairs dog legged to fit the site. http://tillyweb.biz/pmc/thurgarton061975pmc1.jpg

 

There are also several on this page http://www.derby-signalling.org.uk/Newark.htm

 

While down at the club last night, I took a look, and there's a nice plot on the new baseboard, just above a canal basin for a signal box. Bending down and peering along the track it has an excellent vire of the 2-4 line points, and a fair vire through to the poistio where we shall be inserting the main line crossover. It's a plan.

 

 

So - with this plan, and with the acceptance Home signal placed safely in the rear of the tunnel, would anyone care to suggest the rest of the down line signalling in this asrea? Or even better, both lines?