There is a chance that your trying to apply too much traction power and the train is wheel slipping ?
Try starting off using just the first or second power notches and then once your up to about 5 mph move the power controller to notch 3 and so on as train speed increases.
Some of the train cabs have a yellow wheelslip light on the instrument panel , If this is lit then you will generally hear a slipping metallic sound coming from the train to indicate that the wheels are slipping and also the speed indication on the hud (top right of screen) will be flashing .
It's a bit like a car where you need to start off steadily and increase speed , If you floor the accelerator then the car will probably wheel spin and paint part of your tyres on the road
DCC is set up basic like a train controller , trains have a bit of inertia but if you suddenly shut off power they generally don't coast very far .
Cab mode is set up to mimic real life Train driving (not completely accurate compared to the real thing but it's getting better over each subsequent release of Trainz) .