 Diesel-electric locomotives may not always be considered HEVs, not having energy storage on board, unless they are fed with electricity via a collector for short distances (for example, in tunnels with emission limits), in which case they are better classified as dual-mode vehicles.For large boats that are already diesel-electric, the upgrade to hybrid can be as straightforward as adding a large battery bank and control equipment; this configuration can provide fuel saving for the operators as well as being more environmentally sensitive.Producers of marine hybrid propulsion include