The light level of a chest within a minecart, both placed by the player and naturally generating in mineshafts, is set to the light level of the player/the block player is standing on.

Steps to reproduce:

1. Load into a creative world and enter the command /time set night
2. place a rail, then place a minecart with chest on top.
3. move 15-20 blocks away and place a torch or other light source.
4. Observe that the light level of the chest inside the minecart changes with the player's.