Don't know officially but the schedule from Whitby to Grosmont is 22 min for 6.25 miles, which is an average speed of 17.05 mph!!
No pun intended, but there's a bit of time in there for slippage I expect.
I filmed the 1400 from Whitby last summer going through Sleights. It's a bit wobbly as hand held
https://www.youtube.com/watch?v=-emluzHFT2o
That was about 20mph I reckon.
Edit, checked Network Rail website for linespeeds. Max is 30mph between Grosmont and Whitby, 20 in places. That was published in 2006 before NYMR started running. So no 50mph running sadly.