There is no average. However, the trend these days is for longer and longer trains. I had one a week ago that was 9,400 feet long. They don't really like a train that long on the line I run though because there's only 2 sidings where it can clear the main track at. Most of our trains are about 8,000 feet these days. As an engineer the weight of the train is more important than the length. The worst are the very heavy short trains such as cement tie trains and ore trains. You have to work a lot harder on a short heavy train to control speed and slack than you do on a longer train. I had what I consider the perfect sized train home last night. It was 3,700 tons and 5,300 feet long. It was a joy to run.
As others have said, it is a case of "How long is a piece of string?".
I assume that you are asking about North America, where a huge amount of freight travels long distances by rail. Each train will have as many vehicles as it needs to have, and the beauty of modern multiple-unit control means that the railroads only need diesel-electric locomotives of quite moderate power, and hitch as many of them together are required to move it, under control of one driver. The only limits on the maximum length of the trains are those imposed by the gradients, length of loops and lay-by sidings, and the tensile strength of the couplers between cars. The fact that much of the North American railroad system is single track, and run commercially, means that it makes sense to run few, but very long trains, with the minimum of labour involved. The down-side is of course that the track is often not built or maintained to standards that permit faster running; there are thousands of unsupervised automatic grade crossings (especially in built-up areas); and the vast majority of the freight cars have an out-dated braking system that calls for immense skills on behalf of the engineers to control their trains.
The length of a train, including the longest trains, may be measured in number of wagons or metres for general freight.
Conventional freight trains can average nearly 2,000 metres.
hard to find a useful average when trains like 101 car coal trains exist
or are you thinking of a particular service?
heres an interesting list of LONG trains
There is no such thing as an 'average' train.
freight train probably over a mile and a half or shorter, but thats what i normally see