Although there’s no sentence like “maximum length of load allowed on rail” in loading regulations of freight wagons (RIV/AVV Annex II), there are some limitations depending on the flexibility of load.
According to Annex II, the rigid loads can put on maximum 2 wagons. Considering the length of platform wagons, 36 meter will be the maximum length to be transported by rail for rigid loads.
For flexible loads like rail, it’s possible to transport longer loads on rail. Annex II accepts goods flexible if they can pass through curves with 75 meter radius safely.
There’s no clear limitation for flexible loads, but in the table for long loads, the last row is for 120-150 meter long loads. In other words, no spects are given in Annex II for loads longer than 150 meters.
The longest load in countries where Annex II is applied (countries which are member of UIC, mainly European countries) is rail. The longest rail produced in Europe is 120 meters. The above picture shows the 120-meter rails produced by ArcelorMittal for PKP (Polish State Railways) and transported to client on rail.