The end of the train has to pass the speed sign for the train to update its speed. Just the engine passing it does not increase the speed, but it will decrease the speed. So, if you have a 5mph then a 55mph then a 5mph and the train has not completely past the 55mph sign when it sees the 5mph ahead it will never do 55mph. Also consider that trains with yellow signals automatically du 1/2 the speed it is assigned. If it is in a 50mph zone it will do 25mph until it hits a green signal or another speed sign. If it is a speed sign, it will do 1/2 that speed. Hope this is understandable, it has to do with all speeds.