On a custom built track, of standard gauge, with no other restrictions applying except that 95 percent of the motive power of the vehicle must be transferred through the wheels to the track it drives on and 95 percent of that power must at an earlier point in the process be transferred via the movement of water steam heated from ambient temperature using combustion-in-place of organic solid-state fossil fuel materials carried by the vehicle itself (and not electrically heated), a speed record can be measured over a specific pre-measured and marked length of rail line, where-over the vehicle must move, and its speed be measured, twice, within a set period of time (usually within a one hour period), in opposite directions, the average of the two runs' speed which will be the recorded speed and the official record if it so happens (to ensure terrain and weather advantages in one direction is cancelled in the opposite direction).
If those are the official requirements, I can comfortably assume that 300 kilometers an hour is completely reasonable and possible for a custom-built train designed to break the record to achieve for only two dozen million dollars using modern materials and technical know-how.