The original idea was for the railway bridge A to have a fairly low clearance over the track below, high enough to clear something like an 03 with a cut-down cab or a Sentinel and some suitable stock. The front of the layout would be worked by these trains, and there would be an exchange operation with a larger 'main line' locomotive on the gradient at A. This approach would also keep the gradients as easy as they could be. The passenger service (from the main line to the headshunt Y) would be a tube stock driving trailer with an 03 or similar, much like the Alderney railway.
Unfortunately, and without really thinking this through, I started laying the track with the 90 degree Setrack bend B on a level trackbed I called the 'datum'. The headshunt at Y went in at the same level as this datum, mostly for convenience "to keep the calculations for the gradients easier" and I suppose an unplanned/unwanted future extension to the right. So, with the bridge A crossing a track itself on a gradient, the bridge had to be rather taller if the sidings at the processing plant (and the NG line too) were going to pass over the Setrack curve at B. And so, the loading gauge below bridge A has ended up big enough to take my standard gauging vehicle, which is a Hornby Mk3 coach.
Rewritten 8th and 25th August and again 27th September 2015
The exchange operations will now happen on the level track at Y, which must be a better thing. The lesser gradient from the quayside up to the headshunt Y (about 1:25) should add some interest to the exchange operations.
I want to put a scenic break at C to separate the quayside at the left from the passenger platform in the headshunt Y at the right. This scenic break could have a restricted loading gauge, so only smaller locomotives can reach the quayside. This is straightforward to do, but the maintenance depot at D will remain only able to handle a restricted number of locos. It seems best to set operating rules, to govern which trains can pass through the scenic break. I shall make the maintenance depot as close to HO scale as I can, but with an entrance big enough to take any loco which is short enough to fit inside.
The other pinch points on the layout are the tunnel portal at X and the road over bridge at Y - both of these will take the BR Mk3. The tunnel portal is at the point of inflection between 18 and 24 inch radius curves and it surprised me how narrow it can be and still let the coach pass through - it is as though many ready-made tunnel portals are oversize.