Just installing a faster motor in the monorail ignores one of the original issues: the gearing is made for slow speeds.  It's like installing a faster engine in a Caterpillar tractor, it wil go faster but there must be a better way. If we want more speed the motor may be the first and easiest item to address but the gains are limited, changing the gearing is the next step to real speed.  The gearing turns out to be fairly easy! 

There is a worm on the motor driving a worm gear at a ratio of 15:1. Above the worm gear and coupled to it is an intermediate gear that drives the right side main drive gear with a ratio of 1.75:1 (24T to 42T) for a total gear reduction of 26.25:1.  By reducing the height of the "bearings" (plastic posts) in the lower gearbox housing we can add different gearing between the intermediate gear and the main gear shaft.  Instead of  1.75:1 we can install up to 1:1 gears, a potential speed increase of 75%!

The speed increase is optimistic because the original motor will probably not produce enough power to run the monorail that fast.  This is where the new motors come in, to boost the power available (but at the expense of battery life).  Still, an increase of up to 30% with the original motor is may be achievable.

The good part is that this modification is inexpensive.  I bought a large assortment of gears (Amazon) that included several pairs that fit in the required space, all for $4! 

Shortening the "bearings" in the lower gearbox housing is best done with a milling machine but other methods may be precise enough.  About 2.5 mm needs to be removed to provide space for the new gears.  The tricky part is removing/pressing the gears on the shaft, if it isn't straight the gears will wobble forever.  A NWSL gear puller/press will help a lot! The main drive gears are pressed onto the shaft another 2.5 mm, the new gear is pressed onto the right side shaft and a 2.5 mm spacer is added to the left side shaft.

The existing intermediate gear is removed and we see it has a square piece to couple to the worm gear.  The new intermediate gear has a pinion molded onto it and this can be milled or filed square to couple with the worm gear.  Some epoxy will fill the remaining gaps.  Put it all together in the gearbox and go.

With 24 and 42 tooth gears comprising the intermediate and main gears we can use any combination that adds up to 66 except there is only room for about a 34 tooth gear in the intermediate spot (and a small piece of the upper housing needs to be filed off for gears larger than 32).  This limits us to 34:32 which should still provide an impressive speed increase.  The gearbox and the inexpensive gears are not always molded perfectly and 34:32 does not always fit well, sometimes a gear that is 2 teeth smaller fits better.  I also found that sometimes 42:42 does not fit well for the main gears either, 42:40 may fit better (41 is difficult to find).

I also recommend an improved drive roller to replace the often out-of-round original, Monorail Modeler's new roller with o-ring (plus spares) is round!  If your monorail has a lot of hours on it the plastic "bearings" may be worn.  Installing bushings or ball bearings in these areas would greatly improve smoothness.

So this gives faster speed and smoother running and the gearbox can be returned to the original gearing at any time.  The next step up is WhisperJet and it is a big step up but at a much higher cost.  You play, you pay, 3.0!  ;)