Shelly,
We've been taking the AutoTrain for the last 6 years and I do believe the tracks have gotten smoother each year, with my last trip (Dec, '05) being the smoothest. The train does travel 60-70 MPH over many portions. Speed feels different to different people and the 'swaying and bumping' can be felt at any speed - albeit a bit more when moving a top speed. Trains do have accidents and derail, so do planes and autos. There are MUCH, much, mush more auto accidents then trains and autos.
We really enjoy the AutoTrain and it allows us to 'wind down' into our vacation. The AutoTrain is among the best long-distance train in the US. There may be delays and there may be times when it arrives early - I've had both experiences.
All of the items that I describe above are typical of traveling on a train. If you've never travelled on a long-distance train before, the first trip might seem a bit weird and you might be inclined to feel less safe, etc because of the bumps, sways, various normal sounds - but after 1 or 2 trips, it all becomes second nature.
Hope this helps!!