I can ask, but I think parts of the M&E are in fact constant tension.
Basically, constant tension means that the system is weighted at one end, so that the wire never really dips, sags, or moves out of alignment. As long as you hold the tension on the wire at a more or less constant, it will stay where you put it, and do so very easily. Also, the wire behaves the same at every temperature.
Normal catenary holds the wire at a fixed tension - that is, they set it when it's put up, and it stays at that tension. But only at that temperature. The tension drops in the heat, rises in the cold. Extremes either way can break the wire or cause too much sag. It's fine for some applications, like slow speeds or terminals, where you can ditch the weight system, though any new construction uses constant tension because it's pretty much standard - the parts for a 600VDC system are catalog items, even in the US (all that LRT adds up!)
IMHO, the bigger limiting factor is the signal system - no ACSES below New Haven. IIRC, 135mph only happens under certain cases.
As for 9 inches? Not going to happen.