probert1 Wrote:Down Valley Branch to Station C
My min SBD for the Branch lines is 772m and max 1025m. Would it be possible to place a main aspect on the Down Valley Branch at say 660m and then place a signal on the bi-directional Down Valley section at around 1780m. Obviously slightly overbraked, but it would allow freight to be run right up to the Switch diamonds and held there. Standage would be limited as Peter previously mentioned. A sub signal provided with the main aspect at 1780m would then allow facilite a shunt move into the station area.
I assume that you are saying the max is due to being 133% overbraked and that the constraint imposed by headway is less onerous. Placing signals as you suggest would be 145% overbraked; yes this is something that could well be acceptable [NB although the Group Standard indicated 133% this is actually often not achievable withou compromising other standards and good practices. NR does hold a general non-compliance allowing, in suitable circumstances, greater overbraking. Interestingly the former standard was 150% which may indeed have been a better balance between overbraking risk and practicability; it is probably more important for spacing along a route to be CONSISTENTLY overbraked rather than limited to any specific value.
I think though that I'd be tempted to place a signal on the DVB that is overlap clear of the single line, say perhaps at 490. This is more operationally flexible, though with a large risk if SPAD not contained within overlap- certainly need to ensure that good TPWS provision. I have already mentioned possibility of a signal at around 1200m, so to get braking distance in then the previous signal would have to go at 425 rather than the 490 suggested- a longer overlap so a better chance of stopping a SPAD so that is good. Obviously there wouldn't be braking at 100km/h to the signal proposed at 1780 but of course in reality there is the 40km/h restriction over the points to observe. Hence if we defined the speed of that section of the DVB between this turnout and the switch diamonds to be 35mph it wouldn't actually impact on the real maximum speed of trains along this section. Therefore I'd be looking to define that as the speed in the down direction from somewhere along the viaduct to the junction at station C. Not only does this get rid of the need to approach release as discussed earlier but also would no doubt give me enough braking at the redefined permissible speed to my signal at 1780. [NB I am NOT suggesting proposing speed restrictions that actually change the speeds that trains could in reality travel; only adjusting the theoretical speed limit to be consistent with that which trains could in practice attain, given known acceleration / braking and existing adjacent restrictions, in this case over curved path of turnouts]
Who says you need a shunt route / aspect into the station; you may have a need to shunt an ECS into the platform but it could use a main aspect as there is no stated or implied requirement that I have found to go into occupied platform.
probert1 Wrote:Station C to Down Valley Branch
A PL at Station C (DOWN MAIN) platform would then permit a shunt move onto the bi-directional Down Vally Branch. A main aspect placed at 1400m would then allow any freight standing to be signalled across the viaduct to the Up valley Branch.
Again why do we need to signal a train onto the DVB when it is occupied? A main aspect would do.
PJW