You can seal up the space and air condition it. This is the sure bet. But of course has a fairly high operating cost.
Can you seal the room well then provide dry plant air with pressure control like those used in clean rooms to keep the rooms a few millimeters above the atmospheric pressure.
I am thinking of a supply fan with an automatic roll type filter element and a smaller lovered exhaust vent set to maintain a positive pressure. The switchgear buildings are free standing and outside air would drawn in. The other idea I have is to seal the room and install ceiling fans to circulate air.
The coal dust is almost like talcum powder and had built up on horizontal surfaces and transformer bracing. We cleaned and tested the equipment after it had been in service for about 18 monthes and there was an inch of dust on all surfaces. Had the primary voltage been 13kV rather than 4160 volts there would have been a flash over.
No matter what is installed there will be constant maintenance of the filter elements.
We used a similar rolling filter system in the steel mill for filtering large motor cooling air.
The best solution would have been the owner not putting the switchgear next to the rail car loading conveyor!!!!
The lowest operating cost (depending on where in the World you are) and also the best way of keeping dust out is to maintain a slight overpressure, circulate air in the room and make air pass radiators with cool water circulating in them so that water transports heat out of the room. This, of course, only if cool water is available at low cost. Sealing the room so air doesn't get out easily makes need for filter maintenance lower.