> What about a mag-lev track to cut friction losses?

That's definitely a possibility. The sled is the single most
important part of the machine and making it work efficiently
and effectively is very desirable. If the weight is borne by
a mag-lev then it only needs retaining wheels, a la rollercoaster,
to keep it on track

> Or have the sled slide by an array of IR diodes and use a
> photosensor to trigger the transition from rearward expulsion
> to forward expulsion for braking ?

Once the sled has reached terminal velocity there's no need to
slow it down instantly. If I can make it loop-de-loop (I'll have to
find out what radius it can follow at 100mph and friction will be
an important factor) then it'll be on the underside, braking section
of track, which won't take up much extra room

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