Remember, Ras, the track has to rise up to the bridge and then back down, and that's a lot of space needed. Also, if you're using standard Playmobil/LGB track, then you're using R1 curves and these are tight and add to the effort needed to get the loco and rolling stock up the incline. You may need to think of either a "double header" (two locos pulling) or, maybe better still, using a "banker" (a loco pushing in addition to one pulling). This will ease the strain on the motor of the leading loco and allow climbing at a reasonable speed with four carriages.