my bachmann modern 4-4-0 runs fine on dc but stops frequently on dcc. what is the problem
Could be that the decoder is getting to hot and switching off to protect itself.
Ton
DCC is much more sensitive to interruptions of power or short circuits, e.g. when going through turnouts, than is DC. It may be a problem with your track, so checking your trackwork for dead-spots, etc., or fitting a "stay alive" element might help. I had a layout built with DC but wired for conversion to DCC, i.e. all tracks live all the time. On DC there was a problem with two turnouts, one of which was a PECO small radius wye, and I think the other was a PECO small radius. Basically with my original DC controller GP7s and up ran through them fine even at slow speeds, but shorter switchers, e.g. EMD SWs and ALCo S-1-4 stalled unless run through really fast. But when I connected a DCC controller everything stalled on those turnouts.
Best of luck solving your issue,
Bill.