The class WDM-4 is the fourth class of India's wide-gauge (W), diesel-fueled (D) and mixed-load (M) series, capable of hauling both freight and passenger trains. Despite the classification, it was never seen hauling freight trains. It is not known as to why IR decided to allot 4th class instead of 3rd as there was no WDM-3 at the time. This was India's first EMD locomotive, designated as GT16. These were the fastest locomotives in India till the arrival of WAP-3. It does not come under the standard diesel classification system of IR. Similar to WDM-2, it could only haul 9 coaches. So for 18 coach Rajdhani Express, there used to be 2 locos of this class. One of them 18001, is preserved at National Rail Museum, Delhi. The gap in road numbers is due to the units delivered in Pakistan. 18004,