Which is easier, pushing or pulling? It is generally assumed, from a person's perspective, that pushing a cart is more easier than pulling one. But why?
Is there any difference in terms of force required to achieve the same amount of displacement?
Or is it merely human perception?
Why is it that almost all automobiles transfer torque to the back axle. But then, why do trains have engines in the front?
 A: When the block is pushed, the vertical component of the push increases the normal force. Hence, there is an increase in friction. 
Now when you pull the body, the vertical component of the pull is in the opposite direction of the normal reaction and hence, reduces it, thereby reducing the friction.
Hence, friction in (b) is less than friction in (a) 
 Pulling a body is easier than pushing
A: The force required to accelerate an object of a given mass by a given amount will always be constant ($F=ma$). The difference between pushing and pulling is that humans are built in a particular way - our muscles, joints and tendons won't make different actions cost the same in terms of our perceived effort, or our energy expenditure (although the useful working energy transferred to the cart will be same).
I'm not any kind of expert on human kinematics, but I expect the reason that pushing feels easier is because the point of contact (hands or shoulders) can be braced up against the cart, allowing you to use your big driving muscles (quadriceps/gluteal muscles) much more efficiently. When you pull, you've got to try to keep your arms rigid to transfer more of your driving energy into the cart - that means you're burning more energy just in your arm muscles than when you push.
As for the axle receiving the torque of a vehicular engine, I'm pretty sure it's far more to do with steering mechanics and manoeuvrability than any kind of energy transfer efficiency.
A: When the body is pushed the weight of the body divides into $F\cos\theta$ and $F \sin \theta$. $F\cos\theta$ is the force acting on the body, while $F\sin\theta$ is added to the mass of the body and the apparent weight if the body increases.
But in pulling the $F\sin\theta$ is balanced with the normal reaction of weight of the body. So no adding of extra mass to the body and the cos component is the force acting to move the body.
A: 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.
