Thanks for the replies.   The gauge of the front two bogies is the same as the others, it is just that they are so loose that they bounce over the points instead of guiding the engine through.   Often the bogies go one way while the rest of the engine tries to go the other with the obvious consequences.

I bought five wagons to match the engines and they also seem unstable due to the lightness of the models.   It is easy to place weights into the open wagons but more difficult with tankers etc.

I have two new DCC fitted n gauge 2-8-0 engines.    My problem is that the front bogie wheels are so light that the jump the tracks at each set of points (not sure the American name for points I think possibly "Turnout).   The engines run fine in reverse though after a time the bogies are off the track.   
Does anyone else have this problem and what action should I take to keep the bogies on the track.   They are two pretty engines and it is a pity I cannot use them.   Any help appreciated.