Hi there. I’m going to respond mostly to bump up your message on top of the list.
The first step I would do is research about what rail activity is associated with these locations, and what type of infrastructure is associated with it. This will allow you to pick themes/scenes and plan accordingly. Passenger service would involve a station, merchandise could involve a large depot, or even a port. Oil tankers would be associated with reservoirs or maybe even a refinery. Ten by fifteen may seem large, but once you start fitting things in, space becomes way smaller. I would say there is room for two small towns, but you will need to use background pictures to give it some realism. I would recommend one small town, and a smallish station at the other end of the room. The rest would be dedicated to industries and scenery. But it’s your railroad!
Once you picked your ‘themes’, I suggest you take a look on the Web or order books to look at various plans. I’m pretty old school, so I studied books for a few months and developed my own plan using pen an paper. There is software you can use as well, but I can’t really comment on them.
You should keep in mind a few good practices in design. The main one is depth: a plan with more than 24 inch in depth is harder to build, operate and maintain. I designed a plan that allows me to easily access track and switches for maintenance purposes. For these two reasons, I opted for a shelf layout. I have plans for a tunnel, but I will be able to access it from behind. You would probably have space for a peninsula. A shelf layout will probably involve a duck-under or a lift-out. There are pros and cons to both, but the lift-out is what I recommend. It’s harder to build (not that hard!), but it is a lot more practical and a lot easier on the back. Entering and carrying stuff in your room is a lot easier done with a liftout.
Anyway, keep us in the loop (no pun intended) and hopefully others will reply.