4 8 4 locomotive wheels

[nwarky]

Newbie here.  A few months ago I located in my attic a thirty year old Bachmann train set my son enjoyed for years to now enjoy with my grandsons.  Deciding to expand the collection I just bought the Overland Limited train set which includes one of the 4 8 4 locomotive versions.  I openly admit I have no clue about the front and rear "floating' 4 wheel sets that will not remain on the rails.  What am I missing?  What am I doing wrong?  Please advise.  I am confident that this question makes me look like an idiot but I am clueless and can't keep the locomotive on the track.  Help is greatly appreciated.

Dean