An improved system and method for laying on the seabed a pipeline and/or installing a subsea riser including one or more accessories are disclosed. In the system, a trolley rail for an auxiliary trolley is provided which includes a retractable trolley rail part supported by the tower via an associated mobile rail support assembly. Displacement of the retractable trolley rail part together with the auxiliary trolley when arranged on said retractable trolley rail part with respect to the tower is allowed, between an active position, wherein the auxiliary trolley is positioned to engage on an accessory in the firing line, and a retracted and non-operable position wherein a clear envelope about the firing line is created.