Just a short time ago I got some drivers-eye view DVD's with London Transport Tube-lines from a friend. This got me quite interested in the tube-system.
So, just like you I searched the internet for MSTS routes, but I could not find anything (only a view trains).
I did hoewever find the Northern Line available for the (free) train simulator BVE (Boso View Express) on trainsimcentral.co.uk, but as I like MSTS better then BVE, I did not like it enough. The route in BVE is very good, but in BVE routes are only driveable in óne direction.
So earlier this evening, I took a Northern Line station from the BVE route, and used is as an example to create a MSTS-version.
I adapted the texture-files from BVE for use in my MSTS-version, so I will have to contact the creators of the BVE-route if I make it available for download.
I make no promisses, and it might take a long time, but I think I am on te right track. SHould it eventually come to a complete MSTS-route, I will most likely offer it on the internet as freeware (it's after all just a hobby, not a job).
Here is a screenshot. It is the result of about three hours work; a half-built station and about 10 meters of tube tunnel.