The 'services' which may be diverted when a tramway is installed include:-
Water Gas Electricity Telecomms Sewerage
the owners of which are referred to as 'statutory undertakings'.

In the case of heavy 'Light Rail' schemes, these diversions are thought necessary in the first place to avoid damage to services by the load and vibration transmitted to them through the ground. In the second place, the repair of the services is easier if they are not located under a tramway track.
There is also a spurious belief in this country that it is impossible to repair a service under a tram track without suspending tramway operations entirely. The fact that this is regularly demonstrated to be untrue in all the other countries which have plentiful examples of tramways, does not seem to have been recognised in this country where tramways are relatively few.

Safety measures include warnings for tram driver to slow down on sections affected by works, a safety worker to warn any underground workers not to stick their heads up incautiously when a tram is passing over and, in extreme cases, diverting the trams on temporary surface track to the other side of the street.

The cost of diversion of services can sometimes be the most significant factor in the economics of tramway construction in city centres. The expense of diverting a few service water supply pipes would be trivial but the work of re-aligning and protecting the high voltage underground cables of a major electricity sub-station will result in expenditure of great magnitude.

In the proposals for Bath, the use of LR55 rail is contemplated. In conjunction with lightweight vehicles, this should reduce diversion of services to a minimum.

More about LR55?
More about Vehicle weight
More about Costs?