I believe trains have engines in the front/back because the material used to build them is better in tension/compression respectively. There may also be some argument to be made about the train being more stable and better at taking corners with the engine at the front rather than the back (it can't buckle this way), according to my childhood train set.
EDIT As for the human aspect, it depends entirely on how you're doing the pull/push. For instance, it is easier to be strapped into a harness which is attached to a sled (or a lorry as you sometimes see on TV) and then pull the thing along than it is to push it from the back - a matter of the mechanics of the body position. Different body positions recruit different muscle groups when one is trying to drive their body forwards. If we assume we are in the optimal body position for muscle recruitment then your question boils down to the question of which pushing/pulling position transmits force most efficiently to the object you're trying to move - i.e. how can we apply force to the object without having to transmit force through our arms or core, where the force would be dissipated somewhat. Further, this optimal body position is easier to achieve when pulling because the weight of the object allows you to lean forwards for leverage.
The above assumes pulling with a rope or harness, if you had to face the object, grip it and then pull backwards I think that pushing would likely be a lot easier.