(curious, as we don't have the snow and ice problem here, but other 'track access issues'). In a gym, I assume basketball court, what do you use for shoes and/or grip-wise? Do you just thoroughly clean the wood floor for say 10 meters, and your clean your shoes for grip? ...so you 1st and 2nd step out of launch don't slip back?
I've been thinking about buying some of these, and screwing them doing to a wood 2"x12" board, maybe 3' long to make some sets of indoor blocks. The end of the board (could be placed up against a wall so they don't slide back ... and/or a 2nd kid stands on them.
Make it similar to this https://www.amazon.com/Port-Pit-Indoor-Starting-Blocks/dp/B0006MN0KE but add some spikes underneath to fix them to track