1. Maybe there was 15' of snow on the ground when the deer was walking along and he thought he'd jump over what he thought was low fence wires. ZAP!
2. The deer was riding along on top of the train...put there perhaps by drunken pranksters, or chased there by hunters. Regardless, the deer got tired of his precarious perch, what with slippery hooves on sheet metal and all. He wanted to jump to the cold, hard ground, but he had enough horse sense to chill on that idea. When he saw the double T-bar on top of that pole, it looked like the answer. After all, he had been watching T-bars roll by all day, and he was starting to get his courage up. He launched, almost landing all 4 hooves on the double T-bar, but the rear hooves slipped off, he was electrocuted and his hooves blew off like champagne corks at a wedding.
3. Some of those country boys up there got one of those big catapults, and, well...when you run out of cars and other inanimate objects, you've got to search for decent payload somewhere.
4. It wasn't deer season. It was shot in the foot. The hunters had to ditch the body somewhere, and they needed a decent alibi. "Hey! How about the power lines?" "Let's cut the feet off so nobody finds the bullet hole! We'll say the train hit the deer!"