What makes a good game?
There are two sets of answers to this question: what you the customers want, and what we as developers want; sadly, they don't always coincide.
Customers tell me they prefer the more complicated locations, often quoting Carlisle, where there's a lot happening. There's also a lot of positive feedback about ones like North Devon and Waverley Route where you have to be awake to what's happening in a wider area. The smaller ones are not much in demand. Another clear preference is for scenarios set in steam days; even the "classic" diesel period attracts much less interest, and the present day even less.
I'll say more about research and sources of information another day, but it's probably obvious that the older the period we're looking at, the harder it is to get accurate information. That's sad, because the lack of availability of all the relevant information is a main reason for not proceeding with a promising idea. So we can easily and fairly quickly produce an accurate simulation of a present-day scenario such as Liverpool Lime Street 2015; gathering the same information for 1958 or 1972 was much harder.
Once the research is done, the programming is fairly straightforward, and a large part of it is routine but lengthy, the time being proportional to the number of screens and complexity of trackwork.