First, consider how ordinary trains turn. Trains have metal wheels which exert force well in the normal (ie, down) direction, but poorly in other directions. Trains also have solid metal axles, which means pairs of opposing wheels turn at the same RPM. Finally, although trains have flanges to keep them from going off the track, any time the flange actually bears force on the rail it causes an enormous amount of friction and wear. To solve these problems, train wheels have conical bearing surfaces. As the train gets off center on the track, the outside wheel will ride up the shoulder of the cone, and the inside wheel will ride down to the smaller diameter portion of the wheel. This makes the train tilt slightly, changing the direction of the normal force, but more importantly it makes the outside wheel travel more distance per revolution, allowing the train to turn, without making the flanges rub against the track.
Low-floor trolleys have stub axles. This is bad not just because stub axle bearings have to be able to take torque, but because the turning mechanism described above doesn't work, and something else has to replace it. Low-floor buses also have to deal with stub axle bearing design problems, but buses are lighter than trains and more importantly they have cylindrical rubber wheels which can exert sideways force without undue wear.
All of which is why the Silver Line is a bus instead of a trolley.