I like the composition. you got really close to them which helps. the one thing to watch for is to not *almost* cut off the tunnel. step 1 foot to the left and get that tunnel in the shot and you suddenly have the tracks serving as a leading line
it's not really under exposed, it is just dark. you'll notice the sun peeking over the mountain with a touch of over exposure there. this is a situation where a fill flash to give some depth to the front of the photo may help, but it may look fake all the same. the ideal, when possible, would be to come back later for different light