Article snapshot taken from Wikipedia with creative commons attribution-sharealike license.
Give it a read and then ask your questions in the chat.
We can research this topic together.
The SU42 is in fact the same locomotive as SM42 and SP42. The original class SU42 were modifications of class SM42 done during the 1970s. This modification involved the installation of a 500 volt electric train supply for carriage heating, supplied directly by locomotive's generator. The class retained the original SM42 serial number, however most of those converted have now been converted back to SM42 or withdrawn as the carriages using 500 V supply have been withdrawn.
The second phase of modification was carried out between 1999 and 2000 from class SP42. The modification centered on the addition of 3 kV electric train supply. The modified locomotives were classified as class SU42-5xx. 40 of the class were eventually modified and they are based at various depots across the network. Unlike previous iteration, SU42-5xx has electric train heating powered by a separate Caterpillar diesel engine, thus not creating additional load on the locomotive's main engine while heating the train.
Built for shunting purposes, SM42, together with its version SU42, are widely used in passenger traffic on non-electrified lines.
Later modifications
In the 2000s, a number of companies started offering comprehensive rebuilds of the locomotive, involving the replacement of the prime mover, generator and air compressors, upgraded driver's cab (that better isolates against the engine vibrations the original locomotive was famous for) and various changes to the body. Newag offers modification to type 6Dl that is equipped with 3 kV electric train heating.
Other modifications do not offer the electric train heating, making the modified locomotive a member of the SM42 class. Some of the companies that offer these modifications are:
.JPG)
SU42 locomotive
