Adam, I use a 400D meself - tend to use the 28-105 lens mainly. Almost exclusively use shutter priority mode (TV). For freight, it's never going to be doing more than 75mph so I'll use 1/500 or 1/640 as the shutter speed. From there it's a case of juggling ISO and F number (aperture). The lower the ISO the better so on a sunny day you'll want ISO200, cloudy days you'll need ISO400, maybe 800 if it's really grim. Also remember the higher your aperture value, the more depth of field you'll get. Stacey of this forum suggests f5.6 as a minimum. As a guide, on a full sun day, I aim for ISO200, 1/640 or 1/500, f8 for freight workings.
Hope this helps!