The problem is that you need very tall tunnels for that. Subway tunnels are usually some of the narrowest tunnels in order to be cheaper to construct and the trains are built to be accordingly small too (an extreme example being the deep-level London Underground lines).
There’s nothing technically unfeasible about it. Underground railways with double-decker trains are a thing already, I commute through this station every day, though usually with a single-decker train; this is a mainline railway served by both single-decker and double-decker trains (the latter pulled by heavy locomotives, you can’t see that on the other photo because that is the other end of the train).
The problem is that you need very tall tunnels for that. Subway tunnels are usually some of the narrowest tunnels in order to be cheaper to construct and the trains are built to be accordingly small too (an extreme example being the deep-level London Underground lines).
There’s nothing technically unfeasible about it. Underground railways with double-decker trains are a thing already, I commute through this station every day, though usually with a single-decker train; this is a mainline railway served by both single-decker and double-decker trains (the latter pulled by heavy locomotives, you can’t see that on the other photo because that is the other end of the train).