Specter02147, for me, the key to a realistic route is "recreating" the process by which real railroad routes were built. That is to say, the vast majority of real railroads were built for the purpose of making money for their builders and investors by hauling something (or some things) from one place to someplace else, as efficiently, quickly, and safely as possible. You've already specified a time (age of steam) and an area (Britain), but you need a bit more detail, in my opinion, before you start mucking about in the landscape. What points in Britain are you wanting to model the service? What exactly are the businesses producing that your railroad is going to be serving? These will all impact how you build your railroad (Coal and ore are quite tolerant of bumpy track, so a line carrying only these products will be more tolerant of poor track conditions, than they might need to be if the area served is a major producer of glass ware; And if the passengers are only enduring a few km ride to the junction, they may be more tolerant of a rougher and slower ride, than if they are traveling up to Liverpool or down to London). And exactly where your railroad is sited will also affect how the line is built. A line in a narrow canyon with steep walls may be much more expensive to engineer and build, than a line in a broad river valley; and the narrow canyon may prompt more consideration of a narrow gauge line than the one in the broad valley.
Of course choosing to model a prototype route can make many of these questions simpler. You already know, after all, what the products were, and where the line was located. And TransDEM, mentioned by a couple of earlier posters, can be a big help. But there are railroads which disappeared long enough ago, that their rights of way have been obliterated, and no trace remains, at which point other methods have to be used.
On the other hand, if you decide to build a free-lanced route (and by the term "free lanced", I mean a route which did not exist, but if it had existed, it would look and feel, and operate almost exactly the same as the route you built), I consider a good backstory to be almost a requirement for a realistic route. The backstory lays out in some detail the imagined history of the route: who built it, and why, and how. One thing that makes railroad routes realistic is realistic names, and the backstory is the place to develop these. Place names are assigned based upon the history of a route. For example, there may be a location on the route where some farmer once loaded sheep onto the railroad for transport. The farm and farmer are long gone, and perhaps all of the appliances for loading sheep, but the place where it was may well still be known as "Jones sheepfold" (assuming the first farmer to use the facility was named "Jones").
ns