Virtually all freight passing through Derby will be North to West. Freight on the South line, towards Long Eaton, is negligible, perhaps non existent. The only real vantage point for the RTC is just past London Road bridge, the bridge will also give you a good view of the West Main and Goods. Just go straight ahead out of the station, up Midland Road and turn left at the top, the bridge is about a ten minute walk.
There's still a reasonable amount of freight going to and from Birmingham, although you'd undoubtably see more passing through Burton-on-Trent (also ten minutes away)
However, if you fancy doing something else and if it's your sort of thing, take a walk into town, past the railway cottages and take a look at vehicles entering and leaving Derby bus station. The regular chaos there has to be seen to be believed! The bus drivers who use it daily regard it as hell on earth and there's an excellent vantage point from inside the indoor market opposite. It's well worth a look to see the gridlock several times per hour!