You're completely wrong. Most diesel locomotives are in fact diesel electrics. I.e. diesel engine, generator, electric motors. Most of those cannot use external power sources (although a few can, e.g. class 73 in the UK).

Certainly in the UK: the most common locomotives such as class 66, 60, 58, 67, 47 - they're all diesel electric.

In the US, same story: for passengers the GE Genesis is pretty big, things like SD90MAC and AC6000CW are big for freight.