Better displacement finder? Lasers/Inertial Sensors
Inertial devices imply an open loop control and all the challenges there. To have closed loop you need a true track position sensor,
Or perhaps more accurately, a true displacement sensor which can be achieved in several ways, e.g. a dummy wheel and rotational encoder, pressure switches to correct inertial tracking, or a track based linear encoder.
Linear encoders use specifically crafted marks that will allow you to determine where on the track you are, some require constant monitoring, that is, they are not absolute measurements but even with this you will be observing real position movement and thus have your closed loop
A wheel with rotary encoder running on the track will also work and simplify the track construction, but could be prone to slipping and same types of errors as inertial systems, a linear encoder on the track can tell you exactly where you are assuming you have been watching it all along.