Swedish rail-freight operations today have a fundamental structure with a locomotive fleet that is practically twofold. Electric locomotives are used mostly for main line service at night and diesel locomotives for shunting and branch line service mainly at daytime. A large part of the locomotive fleet is hence inactive at all times which generates more tied-up capital. With a locomotive capable of running on all lines, this problem can be reduced significantly.

A systems study was therefore made of a dual-mode locomotive, i.e. a locomotive operating on electric catenary systems as well as diesel mode.

With a well-balanced performance along with high adhesive mass such a solution is believed to make a more profitable operation from many perspectives. Futher, the environmental benefits of a dual-mode locomotive compared to a diesel locomotive are clear (especially when comparing with the most common Swedish diesel loco, the class T44). Large reductions in terms of emissions and energy consumption can be achieved. Running time and hauling capacity in the electric mode of the dual-mode locomotive are comparable with more potent 4-axled locomotives used by Swedish and European operators. Careful dimensioning also makes a dual-mode locomotive very cost-effective. The purchase price is estimated to about the same as a modern diesel locomotive with a 1 500 - 2 500 kW engine in the order of 2/3 a modern European electric locomotive. This relation is due to the usage of standardised components that can be used due to the careful dimensioning.

The present study was carried out by studying a number of real cases apropriate for dualmode locomotives. At present these operations are either operated by diesel locomotives running all the way (including a part under electric catenary) or several locomotive types (electric and diesel). The results showed that a dual-mode locomotive has several benefits, foremost in terms of environmental aspects where the emissions are extensively reduced foremost owing to lower energy consumption which is a result of the change from diesel operation to electric. Benefits are also due to better efficiency and exhaust emission control.

Operations that today demands several loco types, electric and diesel, dual-mode locomotives will in many duties reduce the overall running time as a result of the rationalisation. Another positive effect of dual-mode locomotives can be higher rate of usage of locomotives where the benefits are adopted.

Other aspects of the study was an analysis of actual train weights showing that operators typically run with just about half of the desired train weight (the desired is often the maximum allowed for best profit). Further, the running times were simulated for the dualmode locomotive (in electric mode) along with the most common locomotives in Sweden and also a modern electric European locomotive. These simulations showed that a dualmode locomotive with approximately 3 200 kW rated power (in electric-mode), 250 kN starting tractive effort and 90 tonnes of adhesive mass had highly satisfactory running times (comparable with the European and Swedish electric locomotives) on lines with varied topographies.

The diesel engine in the dual-mode locomotive will preferably be smaller than in an ordinary main-line diesel locomotive. The reasons is that the need for diesel power is lower on e.g. sidings where the dual-mode loco will mostly run in diesel mode at fairly low speed.

It should be pointed out that the introduction of dual-mode locomotives does not exclude freight operations with ordinary electric or diesel locomotives. For heavy main-line operations over longer distances, in particular on lines with long and steep gradients, highpowered conventional locomotives would still be the most advantageous alternative.