Civil engineering (bridges) question on load...?

[skidderback]

I am looking for a term, the "term" rating or weight. The term is applied to railroad bridges and trestles.

What it is (I dunno enough engineering to ask the question), relative to the load the bridge will withstand in total, let's say 200 tons, but ONLY if the 200 tons is not concentrated in less than a specified linear distance.

As I am trying to remember, this was referred to as the "X" weight or "X" rating. What term is X?

Is that a clear enough question? If so, I'll thank you in advance for your time... if not, then we're both outta luck...