Diesel trains suggestions
As of recently, I'm quite into trains, mostly freight ones, so here's my two cents on the topic.
Considering the T1 train's 10m length, most suited for this role would be shunting locomotives, which aren't powerful enough to haul massive freight trains, but are still strong enough to move a decent amount of rail cars around. My suggestion here would be Polish class SM30 built in the 60s.
For a T2 train, following the game's favorism of American vehicles, the EMD SD45 at 20m long is an obvious pick, due to it's iconic looks.
If there are plans for an end-game T3 diesel locomotive, the best choice would be EMD DDA40X at 30m long, which is the most powerful diesel-electric locomotive ever built on a single frame.