I will not buy this record, eet is scratched.
There are 2 issues with pre-industrial high speed (well 50 mph is pretty high in those times) rail: Energy and tolerances.
Anything moving at 50 mph has to be built to reasonably tight tolerances to avoid destroying itself, and any train moving at that speed must be on tracks which have been carefully manufactured to avoid the rails failing or the train leaving them.
The issue of energy is that there are some energy density and efficiency levels you need to reach in order to sustain motion at those speeds and not spend more time refuelling than travelling. Wood, for example, does not carry high enough energy density to be a steam fuel over continental distances.
The only path I can see from within the given bounds is to instead go to higher technology which can be built such that it is able to avoid maintenance. One option for the fuel there is nuclear fuel, which could be prepared in such amounts that they are effectively limitless, and a priesthood which dutifully obeys the sacred rules which codify operational health and safety. Bring forth the sacred glittering tongs!
The tolerances problem is harder; rails are long and thin and subject to both natural and man-made threats. So a feasible long term solution is an effectively magical mountain which contains an automated system for collecting ore, refining the appropriate steel and ultimately printing rails and other necessary bits. This too would be a responsibility of the priesthood, to honour the deity by keeping the gleaming lines straight and pure and parallel from horizon to horizon.
What's my motivation?
One question that comes to mind is 'why?'. Why would a civilisation leave just a very carefully prepared railway and associated infrastructure, but fail to survive to maintain it? Perhaps an option is something along the lines of an alien race which constructs such things in order to do terraforming in a semi- automated fashion in advance of their arrival. A train would be a reasonable system with which to deliver biomaterial, release CO2 or other compounds mined from the mountain across a significant fraction of the habitable surface.
In such a scheme, it also makes some sense for the alien race to anoint a monarchy and priesthood; by empowering the monarchy over other groups with this ability to travel at speed, the monarchy is dependent on the success of the railyway. Thus they should both dominate the continental political sphere and use part of that power to keep the priesthood in clean vestments.
The gleaming, incongruous, holy rail would be a magnet for unrest.