Out of Memory, as mentioned already. Totally agree with comments about an upgrade core, I'd pay for it. Biggest issue for me are the gaps in British AC traction. Classes 81-85, No AC slam-door stock at all. Modern stock like 323, 331, 332, 333, 334, 345, 357, 2nd gen Electrostars, Desiro City, Aventra and Stadler Flirts. You'll struggle to get far on the rail network without seeing one of the above modern units, it feels like TS is falling behind the times but is also missing a chunk of railway history in NSE land.