Hi Tom,
I would by a 6 camera surveillance system. The monitor will give you the 6 camera simultaneous split screen option or you can select an individual camera also.
The system will also come with a DVR which is the brains of the system and links all the cameras. With that you will have the option of playing back recorded video of your trains.
The systems seem to be coming down in price all the time and you can probably score a closed out item that may not have the highest resolution available but enough for your purpose.
TJ