Transport Canada proposes PTC for Canadian Railways - Trains Magazine

I'm curious what the actual current criteria is for requiring/not requiring PTC on a particular line in the US?

While a majority the big railways' main lines here in Canada are CTC signalled (but some stretches used ABS and an equivalent system to track warrants), there are quite a number of shortlines and other minor lines that don't have ANY signalling whatsoever, existing as purely dark territory. I'd imagine the US is no different.