All Pokémon are set to level 50 on the Battle Subway, unlike previous Generations this also includes Pokémon below level 50.
Source: Bulbapedia/ Battle Subway and personal experience.
So all Pokemon, below **AND above Level 50, will be set to 50.
All stats are cut down to what they were at Level 50 (or raised if below Level 50)
All Evs gained will not be lost or ignored due to level.
If you fully EV trained a Pokemon and it is at Level 20, it will keep the EVs and its Battle Subway Stats will show them.
Also, the stats you see in the Battle Subway will be the stats your Pokemon's stats at Level 50.