Dual-mode locomotives are heavier, but weight is actually an advantage when it comes to freight rail. A freight locomotive needs to be heavier than a passenger locomotive to help increase its tractive effort (measured in pounds or kN, different from horsepower). Ultimately, it depends on the unique conditions and traffic of the line if dual-mode locomotives are beneficial. Diesels can still operate on electrified lines, they simply do not pull power from the overhead wires, instead burning diesel to generate power. There are no clearance problems for diesels to operate on electrified lines.
A dual-model diesel-electric locomotive has two separate power plants: diesel electric and all-electric. This gives the flexibility to use all-electric mode with pantograph on track with an overhead catenary, and also operate in diesel-electric mode on track with no electric catenary. The main disadvantages of dual-mode locomotives are that they are more expensive to build and mechanically more complex, resulting in higher maintenance costs. They also lose some energy efficiency by carrying around the weight of one type of unused power plant (electric or diesel), while using the other.
Existing dual-mode locomotives designed for passenger service include the Bombardier ALP 45 DP and the EMD DM30AC. The 2012 SCAG freight rail electrification study concluded that the Bombardier ALP-45DP was the existing dual-mode locomotive that could most easily be converted to freight operation in North America13. These dual-mode electrics were built for New Jersey Transit and Montreal’s Agence Metropolitaine de Transport. In all-electric operation the unit has a maximum power of 4,000 kW (over 5,000 hp) and a starting tractive effort of 316 kN, while in diesel mode power is reduced to about 3,100 kW (4,200 hp). The ALP-45DP units, at over $10 million each, were more expensive than a comparable all-electric or diesel-electric locomotive, and have spent a lot of time in the maintenance shop with "teething troubles". There is limited experience around the world with freight dual-mode locomotives. Despite having more limited fuel capacity than a regular diesel-electric locomotive, even with electrification of main-line track, dual-mode locomotives could find application during electrification construction and maybe in non-electrified yards and sidings.
There also could be plenty of room for hybrid locomotives that could benefit from batteries- not only electric- diesel hybrids made more efficient from on-board batteries (like a Prius), but also electric locomotives that could use both batteries and overhead catenary. The battery/capacitor-catenary hybrid concept offers many opportunities: for areas with relatively short distances of incomplete catenary infrastructure (bridges/tunnels that have clearance issues perhaps), as well as backup even with 100% catenary system. The new "battery-trains" in Japan and Europe are of this type: