Absolute Block Principles, a simple introduction. 

Black and blue levers were also used on mechanical signalling systems where the mechanism for changing the point also locked it, as can be found at Orton Mere on the Nene Valley Railway.

You can still find the pre-groupimg mile posts on some line. The NER used posts with points to indicate the quarter miles - one on top for 1/4 mile, 2 points (one in either side) for half mile and all 3 points (top and both sides) for 3/4 mile. Where the lines of different companies ran parallel to each other the lines would have their own mile markers with co many boundary markers. You can still see Midland Railway boundary markers on the ECML in Peterborough between the former MR tracks that go to Leicester and the former GNR tracks of the ECML. However, the ECML down slow is also the, IIRC, the former up line to Leicester. PS that was one of the more succinct definitions of of Up and Down I've heard in a long while.

Train reporting numbers can be a minefield as each region used letters internally for locations on their own regiom and a neighbouring region would use them for their own internal destinations, along side the inter regional letters. What was worse in the early days the letter A could refer to trains to and from London. In some areas, like Teesside, the letter assigned to a freight train referred to the destination between Tees Yard and Skinningrove steel works independent of the route taken (prior to the 1960s there were 2 routes). These letters had to be displayed or removed when passing Tees signal box. M would normally indicate a train heading the the Midland region, but east of Tees signal box it meant a train heading for Cargo Fleet Inner Junction or the Eston branch. Theyvwere still in use into the 1980s but as the industry on Teesside reduced these headcodes were no longer needed and they used the letter indicating trip workings. The letters used were A, K, M, S, g, P, J and O. In the days before the alphanumeric headcodes lamps were used for tye same purpose the equivalent lamps to the letter M was centre and above left hand buffer. As for the numbers 00 was also used for certain classification of trains (mainly see on parcels trains) and for the following destination letters F, G, X or Z on the Eastern Region (and maybe on other regions well). Limit of shunt and the Barrowmore MRG have documents available for looking at for more details. If the headcodes starts 0 then the next letter indicates the region or district a light engine is travelling to and the last 2 numbers indicate the shed it is going to, for example 0N20 indicates a light engine movement to Gateshead depot. When the locomotive is heading from a depot to work a train the headcode starts with 0 but the rest of the headcodes is the train headcode do if the train headcode is 1A34 then the light engine to work thus train would be 0A34 (the empty coaching stock movement would have the headcode 5A34.

Green lane signal box (and similar situation on others), why do signals 2 and 6 have caution signals (1 and 5) but signals 4 and 7 do not?

This video should have 2 likes from me, but won't let me.