First Question, What size train?
The train will end up being about 10" long, 4" tall, and 2.5" wide as the post said. The walls could be as much as a 1/4" thick for those dimensions.
Second question. what proportion of the time do you intend the train to be running?
Using those panels, the answer will probably have to be very occasionally. Solar panels seem to under-deliver for me, but that may be because I live in the Northwest of England where it rains a lot.
Back to first question, do you have a multimeter, so you can check how many mA the train's motor draws?
What do you mean by "very occasionally?" I'd like to run it as much as possible, but I know the solar panels might not supply what the motor is taking out of the battery (which is okay).
I do have access to a multimeter, but I haven't selected a motor. I don't know what kind of RPM/Torque I need, nor what my power system can support.