I do that all the time now. It's quite fun and challenging to fit in a rail line that might have, or could have existed. Keeping the line convincing and consistent is a big part of the challenge. it's also a bit more interesting building the route around an existing terrain. Things come into play here that are not there when winging it with topology tools. If there are hills, do we tunnel through them, or do we go around them. Ridges and valleys need to be conquered. It's a lot more work, but the results are rewarding.
My in progress Cape Anne Regional Transit, for example, is based around a DEM I generated of the Cape Anne area in Massachusetts. George Fischer's Gloucester Terminal is based on the same area as well, and so is my Gloucester Terminal Electric, which is actually his route plus some expansions. I expanded this one by adding in the Eastern Point branch with trolley stops in Wonson Cove, Southeast Harbor, and finally Eastern Point. Tacking on the DEM to this was a lot more difficult due the differences between old HOG routes and TransDem generated routes. It probably didn't help either if the HOG maps was chopped down too in some areas, making aligning impossible. In the end, I winged it and blended in the coastline. When viewed, no one know there's that difference until I point it out. This line, like the rest of the system runs under catenary for the passenger trains with freights being dieselized. I use the premise here that the old freight motors were in need of repair and the company felt it wasn't worth repairing them. The whole system was set to be dieselized, but the local transit company took over the passenger service.
Using the DEM and topographic maps, I put in the existing railroad, the former Boston and Maine, Gloucester and Rockport Branch on my CART route, then proceeded to add in the other branches to non-existent terminals. I will say that there was once a number of old quarry railroads in the area, and the premise of my Annisquam to Pigeon Cove line is for that reason. The old ROWs are still there today in real life, but the tracks of these mostly narrow gauge lines is long gone for at least a century. This line actually runs beside Route 127 and has stops in Riverdale, Riverside, and Riverside Station. It was Riverside Station that got me curious about this area in the first place. Today the old quarries have become reservoirs and lakes with parks around them. In my world, I've connected up a line to the much bigger ones including Baystate Granite, and another up in Pigeon Cove. The Rockport terminal is met by a branch that comes up from East Gloucester. In real life there was once a narrow gauge line built to, again, to haul granite and stones. The old ROW is obvious and appears as a straight line out to the beach. When the houses were built in the early 20th century or before, they allowed for the ROW so there's that distinct space between them and their back yards. In my world, this line is connected to the East Gloucester mainline, and it too becomes a busy branch to Long Beach.
Eastern Point too is featured prominently in both of my versions. Here I put in a branch to a port that doesn't exist. What was once a Coast Guard Station back in the day and marked still on topo maps, is a sea port with some tracks out on the piers. On my Gloucester Route, the trolley cars run on the mainline and the port-side branch peels off and down around the town to the shoreline.
Taking this one step further, I added in branches to a couple of other areas and a busy branch to Wingersheek. This particular line is double-tracked just like the other parts of the mainline and plays an important part in funneling in traffic to the Gloucester terminal which interchanges both freight and passengers with the B&M at Gloucester. In real life, where I have turn loops on my old Gloucester Terminal route, there was once a substantial freight yard and a short branch into downtown Gloucester to Market Street. Today that branch is long gone and the yard has been replaced by a Shaw's Supermarket or Stop N' Shop.
This as you can see get's quite complex. The other part of the challenge is putting in the lines so that they are convincing. One of the biggest challenges I find is putting in the tracks, outside of the choice between tunnels and bridges, without breaking roads as much as possible through areas that never had a railroad. The Eastern Point branch, for example follows the real life Route 127a out there. In real life this area is a bit snooty with the big mansions, so I wanted to feature those as well as the trolleys pass through Southeast Harbor, passed some horse farms, and on to Eastern Point where I turned a rather rural area with big mansions into a small town. The line too, being a trolley or interurban line, isn't quite as flat as a heavy rail line would be so the grades are a bit steeper. Being a former interurban too, means the freights are shorter, besides it's only a boxcar or two out on the branch anyway where the car is exchanged at the fishing piers.
The thing is use your imagination and have fun. If it doesn't work, the only thing lost is some pixels, bits, and time unlike a real model railroad where the plaster, track, mess, and cost of materials and kits comes into play. This is by far the best feature of Trainz. We can not only take an existing route, outside of the built-in stuff, and modify it to our heart's content, we can do this without wasting anything other than our time.