The Bexleyheath line runs for 8 miles (13 km) from Lewisham to Dartford in Kent. It separates from the North Kent Line just to the east of Blackheath Station, and rejoins the same line just south of Slade Green near Dartford. The route is only 1 hour and 17 minutes so it is very useful. The southeastern bexleyheath line has the class 376’465’466 and 707 trains running on the line. There are 465/0s and 466/0s there are even 465/1 units.
Not sure where you got your miles or times from... https://www.realtimetrains.co.uk/service/gb-nr:W72415/2022-07-07/detailed Lewisham to Dartford is just over 11 miles and takes 26 minutes (all trains are all stoppers on this line) You could literally do the whole North Kent Lines network including Sidcup, Bexleyheath, Woolwich and Greenwich Branches and the termini at Cannon Street and Charing Cross and still be only about the 50 mile "self imposed DTG route length limit" We have 465s in game (wrong subclass but close enough), would need 466 (essentially cut down 465s), 376 and a 700 version when that comes along (I'd suggest the 376 as the included unit if this line was made with the 465 as a required DLC for timetable purposes even though they run by far the most services on the lines)
And that right there is the problem with how rolling stock is packaged in the game. Loco Add-Ons will completely lose their value if included in the core of another route which means a number of potential routes which might rely heavily on rolling stock from Loco Add-Ons won't make sense to create. I sincerely hope this is something on DTG's long-term agenda to resolve, the problem is only going to get worse if Add-Ons continue to be handled in this way.
Or just simply make it like sehs where if you own the 465 it appears in the route, if not it don't. Or they can just do what they do with the 375 to a 377 or a 387, 376 etc.... 465 to a 465/9... A route like this has great potential. I guess it never came to tsc because there's to much traffic on the line. But tsw don't suffer from the same problem.