Jumping on a train depends on the location and direction of the train. It doesn�t depend on the speed of the train, certainly if the speed is less than 1000 kilometres per hour. You land in the same place on the train whether it is moving or not.
If you jump straight up you will land about 10 microns to the West of where you started.
So if the train is travelling West you will land nearer the front. If it is travelling East you'll land nearer the back. You'll land a bit towards the side if the train is going North or South.
This is because the Earth rotates West to East. When you jump vertically your angular velocity decreases in order to keep your angular momentum constant (as you are further from the centre of the Earth) so you will land slightly to the West.
This assumes a British train. The distance will be about double for a Sudanese train, and zero in Antarctica.
Also, you must stay inside the train so that we can neglect air movement, and you must be perfectly symmetrical � so don�t puff out your breath to propel yourself backwards.
Back to the truck of birds.
If the truck is sealed then there are only two ways that the birds can affect the truck. They can perform repeated movements where one move helps the driver and the next move hinders him. Examples are: all flying to the front while the driver keeps his brakes on, then all flying to the back while the driver accelerates up the hill; or all repeatedly taking off and landing together as suggested in a previous post.
The second method is even more far-fetched. The birds need to link wings and swarm very quickly around a central pole to form a gyroscope. The rotation of the Earth will then generate a miniscule force on the truck. If the direction is correct, and if the truck is very, very close to being able to climb the hill, the extra force could in theory, but not in practice, be sufficient.