The <a href="//commons.wikimedia.org/wiki/London_Underground" title="London Underground">London Underground</a> uses a 4-rail system in which the electrical return is isolated from the running rails (the rails used by the train's wheels). The arc seen here on a District Line D Stock leaving Ealing Common Station are quite normal and occur when the electric power collection "shoes" of a train that is motoring (i.e. drawing power) reach the end of a section of electric power rail.
Taken by SPSmiler
<span class="int-own-work" lang="en">Own work</span>