Bi-mode argument rumbles on

The bi-mode debate rumbles on. For decades the economics of diesel versus electric was argued

1. The overall thermal efficiency of diesel was around 30% (depending whom you asked) whereas electric was around 20%.
2. Electric trains and locomotives were less expensive in first cost than their diesel counterparts.
3. Electric trains and locomotives were less expensive in running cost than their diesel counterparts.
4. Electrification was not worth while unless traffic density was at a level sufficient to justify the installation of the fixed equipment.
5. Electrification caused less pollution at the point of use.
6. Electrification gave flexibility as to energy sources.

This probably lists all the most important factors. Applying figures to them is uncertain, but however it is done, the end result is a "triage". There are routes which must be electrified, routes which are definitely not worth electrifying, and routes where it makes little difference either way.

The other consideration is the break-even point for haulage by diesel locomotives rather than DMU, which most authorities suggest is five or six vehicles, and the end result is that the system will have routes which are not electrified throughout their length and are unlikely ever to be eg London to Cornwall. On such routes, until someone thought up the idea of bi-mode, it was accepted that diesel running under the wires was inevitable. I suspect the energy costs of carrying around diesel engines under the wires, and transformer packs off the electrified routes, is not particularly significant in the overall costs. And until recently, it was always argued that running a diesel powered train results in a saving of energy compared to running it under electric power!

The biggest costs on the railway are track costs, partly related to weight, the capital costs of the rolling stock, and staff costs, incurred in operating and maintaining trains. Energy costs are a relatively small item compared to the others.

From this, the following strategy emerges

• keep old rolling stock in operation for as long as possible and do not withdraw it unless maintenance costs are starting to climb or it fails to satisfy requirements.
• KISS - avoid complexity unless it is robust and the technology is mature so as to minimise the capital cost of new rolling stock and its subsequent maintenance costs.

On that basis, bi-mode has only a marginal place on the railway. As a mainstream operating practice it is nothing more than a fad, just the kind of fad that is to be expected from theoretically-minded civil servants. It is, incidentally, far from new, having been embodied long ago in the class 73 electro-diesels, a useful fleet which could operate under limited power away from electrified routes.