There are 12 Wagon-run levels in the game. They are all called 'Wagon Training' followed by a Roman Numeral number. You will initially pass these by brute force but afterwards you may need the information below.
Its not immediately clear but the goal in these is to survive through the obstacles and stop in the end zone.
As a general rule you will want, a gun, a battering ram and a jump.
The most important of these is the jump and is also the one that costs the least weight.
Generally aim for the power bar at slightly less than half of the green 'optimum bar' as it tends to overshoot slightly.
The guide for picking your items depends on the level. Generally, latest unlocked ram and gun, depending on the soldiers. Tier 1 soldiers are only affected by a tier 1 gun and so forth. The only issue you might have here is in Autumn and Winter where you have tier 2 and 3 soldiers. Here you want to prioritise the tier 3 as they do the most damage to you. (*they look like knights).
Another hint is choosing your wheels, if you find yourself going too slow or fast, your wheels have an affect. Romeo wheels reduce the effect of Tar, that's the only real important one.
Hold down on your item choice to see the effect it has. This I didn't learn until right at the end of the game and may well be the most important piece of information!
Hope this helps :)
Before you start the wagon run