The cap was primarily for radio suppression and running without a cap on DC causes the armatures to flash over which rapidly destroys the brushes.  

I have about 10 Triang Transcon diesels and various X04 beasties we use on a floor layout on holiday, as well as more detailed Triang based locos / chassis and  I just use a big ceramic cap across the pickups (or Brushes)  A jinty would easily take a pair one in each tank.  Maplin used to have goody bags of assorted ceramic caps, I just used the biggest. Highly scientific and all that.  I have used Relcos with good results for 30 odd years.  Weighting locos works well to in prove pick up, DCC ones are inevitably lighter than DC and I sometimes run a DC garden line in the rain with no pick up issues.

I too would like to know what capacitors to put in DC engines. I brought a Maplins multi pack of blue ones and wired two together a certain way as suggested by some go stop them exploding...but the engine didn't budge an inch so anyone have a more scientific and specific answer!?