Slipping drivers question for a 30 inch gauge 2-6-6-2

Bachmann has announced it will produce a model of an actual 30 inch gauge 2-6-6-2 that was exported by Baldwin to I believe the Dominican Republic for service on a plantation there.

My guess is the articulated design had more to do with axle loadings and perhaps curvature than need for "super power" as this is a small locomotive.

If I could ever manage to figure out exactly how to load a photo I would post one. However, if you must see a photo, go to Bachmann's web site and check the announcement for new models at the NMRA convention.

Now for the actual prototype question. What would cause a locomotive like this one to have a slippery front set of drivers? Would it be having to climb a grade, the need for more weight on the front set of drivers, slippery track conditions, simply starting a train that is a bit beyond its capacity, or all of the above?

Last question. Why is the front engine more slippery for an articulated than the rear engine?

Thanks!