One relatively recent video card should take care of two screens. Since you're talking about trains, which don't exactly rocket around, there are USB to video adapters that cost around $20. So you could have a 6-screen setup for around $120 plus the cost of the screens themselves.
I had a similar setup for a financial display: whatever came with the computer plus $40 gave me 3 screens. We fitted out an entire office like this. The trick is to not tell people you've found such a cheap solution. If you do it right, the mounting brackets for 6 displays cost way more than the USB display adapters.