BCL
Engineer
I'd recommend this (I have one myself):
http://www.amazon.com/Belkin-Mini-Surge-Protector-Charger/dp/B0015DYMVO
This one is a surge protector, so it will protect against spikes in the power supply since the power supplied by a locomotive can be unpredictable. You don't see it in this photo, but there are two USB power-only outlets, and the plug swiwels 360 degrees if you need to route in strange ways. I've used it on some trains under tables, where I had to turn the plug to get it to fit.
They don't seem to have the even cheaper "Amazon Painless Packaging" version I bought, but it's less than $12 right now in retail packaging.