I have a niagara 4-8-4 i got with the overland express set.  it ran well last yeaR BUT THIS YEAR and RUNS A SHORT DISTANCE AND STOPS.  The push rods from pistons may be a bit bent on one side i coulgently put inbto place a little better.  it may have been a while since it was oiled and greased so I  wonder if that would help.  I have products made for these.