I had that happen once or twice during the beta where I'd lose the speed indications and everything, and the only way to get them back was to stop the train, remove the driver, and then add a driver back in. I also ran into a graphics issue were some of the numbers disappeared from the speed indicator. The single digits were okay, but anything above '9' was 1_ (_ = space), etc. That resolved its self with no intervention on my own so it must've been a code problem that got fixed in the final release.
I don' know what I did but speed indicator disappeared, can't find where to turn back on. Driving blind.