When the Metro extension was constructed, the signalling systems were completely replaced and altered in order to ensure the maximum level of protection from collisions between the comparatively lightweight Metrocars (Which don't feature even the slightest of crumple zones, with seats alongside the drivers' cabs) and traditional heavy rail stock and heavy freight trains.
Four aspect signalling was installed over the Pelaw - Sunderland section of the line when the Metro was introduced, replacing what I remember being three aspect signalling, in order to give trains greater warning of occupied sections up ahead and more opportunity to brake, as well as to allow the increase in frequency.
There is also a delay in the change in signalling aspects when a train leaves one section and then enters another, so that the signal protecting the entrance to the section that the train has just exited remains at red for a little while after the train has passed into the next section. This is in order to increase the distance between the Metros and the train behind and to help insure against collisions in case of a serious SPAD. You can watch this process in action at most of the stations on the Pelaw to Sunderland section (Excluding Stadium of Light and St Peters).
Drivers must also perform a radio changeover from Metro control at South Gosforth to Network Rail, presumably Tyneside ICC, at Pelaw when heading towards Sunderland, and vice versa when travelling in the oppostie direction. The reporting code of the Metro trains also changes when moving from Metro infrastructure to Network Rail lines, with the Metro's three digit train reporting numbers (Such as "106") changing to a 2I?? headcode where the last two digits correspond to the Metro reporting number, so in this example it would become 2I06.
Differential speed limits have also been introduced on the Heworth - Sunderland extension since the Metro was introduced, permitting different maximum speeds for the Metros and for heavy rail stock. The Tyne & Wear Metro has always displayed line speed limits in kilometres per hour on it's own network rather than the more traditional miles per hour employed by Network Rail, so on the Sunderland extension the speed limits for Metro trains are shown in both miles per hour (On a circular sign) and in kilometres per hour on a hexagonal sign positioned lower down. The Tyne & Wear Metro used to display line speed limits (in kph) on circular signs across it's own self-contained network, but I think that these have now all been altered to hexagonal signs to match those on the Sunderland extension to avoid any potential for confusion.
There are some examples of the differential speed limit signs used on the Metro extension towards the bottom of this page:
http://www.railsigns.co.uk/sect13page5/sect13page5.html
Now you see i need a reference :/