[Triplex]

If you look at "Tractive Effort" figures even the smaller diesels and electrics appear to be much higher than most steam locos but I don't think it can be as straightforward as that.

Speaking from a North American perspective, but I expect it's the same everywhere...
Diesels tend to have higher tractive effort than steam locomotives that could produce the same horsepower at speed. It's not hard to see why: diesels generally have all axles driven, and their wheels are smaller (thus more torque). Thus, their operating characteristics are different. For example, replacing a steam locomotive on a fast train with diesels would often mean that it no longer needed a pusher on a grade.