Each type still has to be tested on the routes it runs though, so neither the original or Hitcahi equipment on the Networkers will have been tested in the South Central and South Western areas. In the same respect, Electrostars will not have been tested in the South West area (other than where there is overlap) or Desiros and Junipers in the South Central and South Eastern areas. Testing could be done, but it's expensive and time consuming, so not worth while simply incase "the need arose". If there was ever a desire to move the Networkers from South Eastern then oviously testing would be done, and most likely little if any work would be required. Same thing often applies with gauging, a particular type of stock might be banned from a route not because it's out of gauge, just because it's never been checked to be in gauge.
The simple technical reason for the need to immunise signals is that on DC lines, track circuits use AC to seperate them from the return current. The problem is the modern trains have AC motors so generate AC return current and that can interfear with the signalling. The basic solution is to make sure that the track circuits use a frequency that the trains are unlikely to produce and to stop the trains producing that frequency (at least at a high enough current to be detected by the track circuits). It's also a problem in the winter as under abnormal conditions, such as conductor rail icing, the trains can start producing dangerous frequencies, so the control systems shut them down.
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