You probably think the prior answer wasn't very helpful? Well you get out what you put in, if you'd have said from what DLC the P42DC Amtrak was purchased with (came with) then it'd give people a lot more to go on. Three key (important) pieces of information are required. Purchased from (i.e. Steam) Developer name (i.e. DTG) DLC name (i.e. Big noisy American diesel pack) You help yourself by helping us to help you, that's how it works.
Firstly do you have the right software to edit the textures? You need TS Tools (although I think other software exists) to convert from TS' proprietary format. You also need to edit the BIN files to make the textures their own separate train rather than a direct texture replacement. And as Reef said, which P42 is it? There's quite a few available and each one will be in a different location.
