To begin with, I started with pictures. There are a number of color pictures of New Britain in a number of books, and here are a number of other ones from slides and video captures. As always, you can click on a photo to enlarge it. There are a lot this time...
Note the many variations in color and texture - all of these photos were taken within about 5 miles of each other.
These are on the Highland Line.
Highland Line (near Plainville), note the amount of dirt.
Wooster St. New Britain
These next three are westbound into New Britain Yard c1946
(Paul Wales video)
These three are from the same location, c1953.
(Kent Cochrane video)
Black and white photos can also be helpful. Although you don't get the color (obviously), there is obviously a lot of dirt mixed in with the ballast in these photos:
1956 (photographer unknown)
Kent Cochrane 1946-1947
So that gives me a starting point for color and texture. In New Britain itself (particularly the yard) there's a lot of dirt, some cinders, and some stone ballast, along with weeds. In addition to the historical photos, I've also visited locations, particularly New Britain, that look similar to what's in the pictures. Obviously there aren't any areas using cinders anymore, but I think the appearance of cinders and dirt is pretty similar to the small pieces of asphalt and dirt you see along the side of the road.
So I've been using a mix of Scenic Express cinders, light limestone (#40), dark limestone (#40), charcoal sanded grout, and genuine New Britain dirt. I'm using Pledge Floor Care Finish (aka Future) as a glue. The track is painted with Rustoleum Camouflage paint, then weathered with Pan Pastels.
The two mainline tracks are at the top with more of the stone ballast.
The turnout is the Berlin Line and is cinders and dirt.
Again, the two tracks to the left are the main, the platform track is also the Berlin Line.
Compare to the photo of the L-1 and the man walking across the track above.