Some train aren’t showing up, such as the 1938 stock. There another issue where I can’t edit the consist, such as when I spawn a class 710 it won’t let me remove any carriages. I’ve placed support tickets through my alt but only received a “This is a known issue we are looking into it.”
Some trains like the 1938 Stock have been missing since formation designer first released, we don't know why they don't want to add it Being unable to use individual parts of a multiple unit seems to be a thing they have started doing since TSW 5, we don't know why they have decided to do that either As a workaround you can find some missing things on creators club, like the individual parts of the 710 or the 38 stock, and then add them into formation designer that way. There is a bigger list of missing stuff here