Depends how fast you can push.
The line speed for the R on that line is around 110km/h. So depending on the weight, crews and weather, id say expect it around 100km/h.
I was under the impression that all heritage trains ran at 80kmh? It is at 100kmh because 707 operation use steel cars or is the 80kmh speed limit just for running on the RFR?Non RFR lines, heritage trains are able to operate at line speed to the speed the rollingstock allows.