Power draw on the modern DC network—how is it evaluated?

[M-Train]

This used to be defined by conductor rail index, but apparently according to other threads, this does not apply to anything AC motored from the Networkers and 373’s onwards: is it now simply a matter of current draw instead, as those threads imply, or is there a third figure to look at which, like the CRI, is power-dependent?

I ask all this because there’s been mention of 73’s in electric mode overloading LMR DC infrastructure back in the day, mainly on the NLL, yet the modern-day Watford DC has no problem with being mostly 378’s, which are twice as powerful.