Why are some EMUs that only work on third rail and have no pantograph to work on OHLE numbered 3xx and not 4xx or 5xx. I thought that 3xx were for AC EMUs and 4xx and 5xx for DC EMUs. Some EMUs can run on both AC and DC and are numbered 3xx. But I thought that DC only units were numbered 4xx or 5xx. But some are numbered 3xx. Why?