Older supplies, such as found in a 286, do not yield very much current on the 12-volt output. Usually no more than 1, maybe 2 amps, so they aren't a great improvement over wall-wart supplies. The 5-volt outputs are much more robust, with 10 amps or more being common. That makes them very useful for lighting and timing sensors, etc.
Newer supplies often have circuitry that reacts negatively to the wild fluctations in demand that slot cars produce even though the 12-volt outputs may typically yield anywhere from 3-10 amps. Some supplies will even oscillate between on-and-off or just plain get flaky when the load on them swings as it will in use with slot cars.
It depends on the specific supply, and there are literally hundreds of different ones out there, but it is hit-or-miss as to whether you can successfully use one.
By all means, try it if you have one laying around. But don't expect it to perform like a proper power supply.
Even hot 'HO' motors will tax a PC supply. EM's trick will help, but reverse EMF and the constant demand fluctuations will tend to affect cheaper supplies. I played with this stuff for years and gave up, eventually building a massive linear supply that I still use.
Good supplies aren't cheap, but now you can get one for about the price of two or three cars, gain the advantage of variable output voltage.... and never worry about power again.... ever!
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