Streetluges like skateboards use trucks, the trucks provide steering, axels for wheels to be mounted on and something to fiddle with when you get board.
Some luges use only 2 sets of trucks, one at the front and one at the rear, others use 3 sets of trucks two at the front and one at the rear, the extra set at the front help a rider get extra grip in a corner at speed, as well as offer an extra fail safe if any of the wheels fail.
In more recent time more riders have been using 4 sets of trucks, two at the front and rear, known in the sport as running "quad trucks"
it all depends on your own personal preference what setup you choose to use, you will find that some setups suit your style of riding better than others, it's just a matter of trying to see which you feel most comfortable with.
There is much debate about the effects that having extra sets of wheels touching the ground has upon a lugers top speed.
Does the friction from the bearings and wheels reduce the rolling speed?
Some riders believe this to be the case, so they raise the 2nd set of trucks (the trucks closest to the pan) about 1mm so they only touch whilst turning, therefore adding grip in corners and a longer wheel base on the straights.
This may also prevent the two trucks working on different turning arcs going through a corner.