Marine reel lay method pipeline installation system for laying on the seabed a pipeline and/ or installing a subsea riser including one or more accessories, wherein the system comprises: • one or more pipeline storage reels for storage of pipeline to be laid; • a pipeline launch tower, which is adapted to launch the pipeline in a firing line along said tower; • a pipeline guide which is provided at an elevated position, which pipeline guide is adapted to guide the pipeline from a storage reel over the pipeline guide into the firing line, • one or more tensioners (31, 36), each tensioner comprising a tensioner frame (31a, 36a) and multiple tracks (31b, 36b) supported by said tensioner frame, the one or more tensioners being operable between an active pipelaying mode, wherein the firing line extends through the tensioner frame and between the tracks, so that the tracks are positioned to engage the pipeline and to support the pipeline in the firing line, and a retracted mode wherein a clear envelope is created about the firing line; • a hang off device arranged in the lower part of the firing line, adapted to clamp and support the weight of the launched pipeline in the firing line; • an abandonment and recovery (A&R) system comprising at least one A&R cable and associated A&R winch, and an A&R sheave arrangement (23a, 23b) with one or more sheaves provided at an upper position in the firing line, said one or more sheaves being supported by the pipeline launch tower, adapted to raise, lower and support the weight of the launched pipeline in the firing line; • an auxiliary trolley which is movable along a vertical trolley rail and which is adapted to engage on an accessory in the firing line in the retracted mode of the one or more tensioners; characterized in that the trolley rail comprises a retractable trolley rail part supported by the tower via an associated mobile rail support assembly (51a, 51b), which is adapted to allow for 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 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. • one or more pipeline storage reels for storage of pipeline to be laid; • a pipeline launch tower, which is adapted to launch the pipeline in a firing line along said tower; • a pipeline guide which is provided at an elevated position, which pipeline guide is adapted to guide the pipeline from a storage reel over the pipeline guide into the firing line, • one or more tensioners (31, 36), each tensioner comprising a tensioner frame (31a, 36a) and multiple tracks (31b, 36b) supported by said tensioner frame, the one or more tensioners being operable between an active pipelaying mode, wherein the firing line extends through the tensioner frame and between the tracks, so that the tracks are positioned to engage the pipeline and to support the pipeline in the firing line, and a retracted mode wherein a clear envelope is created about the firing line; • a hang off device arranged in the lower part of the firing line, adapted to clamp and support the weight of the launched pipeline in the firing line; • an abandonment and recovery (A&R) system comprising at least one A&R cable and associated A&R winch, and an A&R sheave arrangement (23a, 23b) with one or more sheaves provided at an upper position in the firing line, said one or more sheaves being supported by the pipeline launch tower, adapted to raise, lower and support the weight of the launched pipeline in the firing line; • an auxiliary trolley which is movable along a vertical trolley rail and which is adapted to engage on an accessory in the firing line in the retracted mode of the one or more tensioners;