Tram Braking Distances under the Influence of MgCl2 as a Dust Binding Agent

Résumé

A field study of friction loss during tram braking under the influence of MgCl2
has been carried out. The work was initiated after a collision in Trondheim, Norway, where a
tram ran into a stagnant bus from behind. During the studded tyre season, an MgCl2 solution
was regularly applied to the street surface as a dust binding agent, and was applied only hours
before the incident. The objective of the project was to determine whether the use of this
MgCl2 solution for dust attenuation may have contributed to a reduced top-of-rail friction at
the moment of the accident. The study comprised of a literature survey, brake tests with trams
and analyses to determine the friction conditions at four different top-of-rail conditions, i.e.
dry, wet by water, wet by MgCl2 solution and dried/moist MgCl2, three braking modes, i.e.
with friction controlled disk brakes, electromagnetic track brakes, and with one of four track
brakes deactivated and three levels of target speed, i.e. 5 km/h, 15 km/h and 30km/h. Around
110 individual tests were carried out. A main conclusion was that the use of MgCl2 resulted in
significantly longer braking distances, especially for normal braking. Dried MgCl2 induced
the longest braking distances, followed by wet MgCl2 applied to the track. The third longest
braking distances went to the track wet by water, and the fourth, with the best friction, was for
the dry track. One disabled magnetic track brake did not seem to have a major impact on the
stopping length compared with all four magnetic track brakes activated. As a result of the
investigation, Trondheim municipality has stopped the use of the MgCl2 solution as dust
binding agent in streets where there is tram traffic.