Usually details of traffic moves are more or less confidential. This protects the railroad of course, so another railroad or a trucker does not get the business, and also helps protect the shipper so HIS competition does not steal HIS business.
That said, in some cases you can figure out fairly easily what you are seeing. If it's a unit coal train the cars are sometimes owned by or leased to a specific utility which can be identified by a check in the Equipment Register. Another example - you may not know the destination, but if a bulkhead flat is carrying white bundles labeled "High Hills Timber Products Pudunk BC" you have a pretty good idea of what's on it and where it came from.
You won't be able to know all of it, especially boxcars, but what was it Yogi said? - "You can see a lot by just looking" or something like that.