A potent combination: tests conducted by Genessee & Wyoming show that a mother/slug locomotive set can be a cost-effective alternative to a conventional two-unit consist

Railway Age, Feb, 2003 by David L. Powell

What has one 3,000-hp diesel engine but the same hauling capability of two conventional freight locomotives? AGP40-3 mother/slug set-two locomotives coupled together powered by one diesel prime-mover. Usually the two locomotives are similar in size, weight, and other general features. The mother unit (the powered locomotive) has some additional hardware to allow for transfer of elcctrical power to the slug, which has no engine. The slug has to be bal-lasted to compensate for the weight loss resulting from removal of its engine and components until its gross weight is brought back to near its original as-built weight.

Economics make a mother/slug set attractive by providing more tonnage capacity for a lower operating cost. One mother/slug set with a single engine can pull the same tonnage as two conventional units, at slow speeds. Recent field tests have shown that at slow speeds and with the addition of high-adhesion technology, a mother/slug set can actually pull significantly more than a conventional two-unit set.

Mother/slug sets have been used in classification yards for about 30 years. Today, however, they are being built for road service because, compared to a conventional two-unit set, they offer fuel, maintenance, spare parts, emissions, and acquisition cost savings.

There are some limitations for road service, the most significant of which is the speed that mother/slug set can obtain while still being productive in terms of tractive effort. Because only the mother unit is providing all the power for two locomotives, its main alternator must provide maximum electrical energy right up to the point of its output limit. The alternator becomes 100% saturated. Typically, top-end speed of a mother/slug set is around 25 mph; modern units are usually programmed to go "off-line" at that mark.

A mother/slug is also usually designed to maintain a nominal loading of approximately 100-300 amps of traction motor current so that the traction motor brushes will continue to maintain the required carbon film on the traction motor commutator surfaces. This film is vital to commutator life and conductivity because it prevents brush arcing (flashovers) that will damage the commutators. When the speed drops below 25 mph, the slug is programmed to automatically restart. The slower it is going the more force (amperage) it will provide for pulling.

Genessee & Wyoming's Ferrocarriles Chiapas-Mayab, Merida, Mexico, built a prototype EMD GP40-3 mother unit and RM-1 Road Mate slug in early 2002 and have tested them in road service throughout the Yucatan Peninsula in areas with 2.2% grades and continuous 18-degree reverse curves. The goal was to build a low-budget slug set and to track in-service performance to provide operating and cost benefit data. The railroad ran comparison tests against a two-unit GP38-2 set. Both consists had comparable total gross weights and horsepower.

Chiapas-Mayab's mother/slug set is capable of pulling 15% more tonnage than the GP38-2s measured from a dead start on a 2.2% grade in 18-degree reverse curves in identical field conditions. This has been attributed to the improved adhesion levels obtained through advanced Dash-3 microprocessor technologies. On other portions of Chiapas-Mayab where there are less-severe grades, test reports indicated that the mother/slug set is capable of hauling more tonnage than its GP38-2 counterpart, thus allowing the railroad to incorporate it into the railroad's systemwide Tonnage Ratings Charts at a 23% higher tonnage rating. In terms of operating costs, the field test results in Mexico are limited in terms of maintenance cost comparisons for a mother/slug set because of the short time in service. However, early indications suggest that so far a mother/slug can cost 35% less to maintain than a comparable two-unit GP38-2 set.