On platforms etc, the electric fields come from the overhead catenary wire (in systems with overhead electrification), and the magnetic field comes from the current loop formed by the supply (the overhead catenary or the third rail) and the return (the running rails or a dedicated conductor).
On board trains themselves, there is still a magnetic field from the loop formed by the supply currents. There may also be magnetic fields from conductors carrying currents through the train. There can also be a magnetic field associated with the motors. This is not usually from the motor itself but from inductors or solenoids in the control circuitry. In electric multiple units, there are motors in at least some of the carriages and the supply circuitry, under the floor, can provide remarkably high fields close to the floor. With locomotive-hauled trains, the circuitry tends to be in the locomotive and passengers are not exposed to the fields from it.
The WHO Environmental Health Criteria summarises the data as follows (reproduced from table 10, page 45 but with typos corrected):
| Table 10. Alternating magnetic fields from UK electrified rail systems | |||
|---|---|---|---|
| System and Source | AC magnetic flux density | Frequency | Comments |
| London Underground | |||
| Up to 20 μT | 100 Hz | In the driver’s cab; arising from traction components and on board smoothing inductors | |
| Suburban trains | |||
| 750 DC Electric Motor Units | Up to 1 mT | 100 Hz | Floor level |
| 16-64 μT | 100 Hz | In passenger car at table height | |
| 16-48 μT | 100 Hz | Outside train on platform | |
| Mainline trains | |||
| Electric Motor Units | Up to 15 mT | 100 Hz | Floor level above inductor |
| Locomotives | Up to 2.5 mT | 100 Hz | 0.5 m above floor in equipment car |
| 5-50 μT | 50 Hz | In passenger coaches | |
Sources of information
Much of the information comes from a paper by Chadwick and Lowes, see below. There is also a Report:
Allen SG et al. Review of occupational exposure to optical radiation and electric and magnetic fields with regard to the proposed CEC physical agents directive. Chilton, National Radiation Protection Board, 1994 (NRPB-R265).
More recently, the Rail Safety and Standards Board have published research on EMFs. This is focussed on compliance with the European Directive but contains quite a bit of general information on fields:
Ann Occup Hyg. 1998 Jul;42(5):331-5. Chadwick P, Lowes F. National Radiological Protection Board, Chilton, Didcot, Oxon, U.K. People on trains can be exposed to static and alternating magnetic fields which are higher than background levels in most homes and many workplaces. Quantification of such exposure may be of interest for epidemiological purposes but it is also important to ensure that exposure guidelines are complied with. This article describes the types of electric trains and trams in use in the UK and the results of measurements of static and alternating magnetic flux density. Many of the data have been supplied by the operators of the systems described. The measurements summarised in this article are indicative of the magnitudes of magnetic field exposures to be encountered on British trains, but without concomitant frequency information, they are not sufficient to allow demonstration of compliance with exposure standards. |