- separate rights-of-way(in other words, tracks shared with freight rail are out of the question)
- broad curves
- elimination of steep grades, whether by earth removal or tunnels.
That same Trib article, interestingly enough, mentions the current speed of current trains between Harrisburg and Philadelphia as being "up to 110 mph" according to a PennDOT spokesman. Again, though, that part of the state is relatively ideal terrain for transport, as shown below.