Electrified Rails are a new feature to OpenTTD 0.5.0 which allow electrically powered train engines to only run on electrified track.
The only trains electrified by default in OpenTTD are the following:
Electrified railways can be built as normal track is, and can also be intersected with normal track. However, electrified trains will not travel onto the normal track. Normal track can be converted to electrified track via the convert rail tool.
Extreme care must be exercised when upgrading ordinary rail to electrified rail as it is very difficult to spot small pieces of non-electrified rail that you may have missed (especially since track with a train on it will not be upgraded). Any electric train will stop and reverse on meeting one of these sections, which will cause major hold-ups on your network.