Inspired by 3d model of Engine Room made by SteveFury i gave a try to do a plan of catwalks arragement in Reciprocating Engine Room. Those plans might look very confusing at first glance, but maybe this small legend will make a situation a bit more clear.
1. Red - catwalks at middle of engines columns. Those include catwalks around the engines, catwalks in engines to service oil supply for crossheads and stephenson linkages, catwalks around Feed Water Filters, catwalk in forward middle area for access to steam valves and catwalk to Engineers Store Room. Catwalk that is running between HP and IP cylinders is on starting platform level.
2. Green - catwalks on the level of cylinders bottom. Those are only in the middle area of the room.
3. Orange - catwalks on the cylinders covers level. Those include catwalk that is running a whole lenght of the engine, catwalks to inspect the balancing pistons on top of LP cylinders valves chests, cross catwalk in middle forward for access to main bulkhead valves of Main Steam Lines and a cross catwalk at aft end of the room with platform to acces Scotland Road on E deck.
4. Blue - catwalks from RED level to Engineers Accomodations on F deck.
5. Dark Grey - stairs from RED level to Engine Room floor plates in the wing areas.
6. Yellow (pure guess) - catwalks for access to auxiliary steam supply valves.
Plans are only a general arragement and might be inaccurate. Though they are my educated guess based on available resources. They are not intended to be used for taking measurements etc.
TOP DOWN PLAN:
PLans used to create those pictures comes from the book 'TITANIC: THE SHIP MAGNIFICNET'.
EDIT: I now see that i forgot to include cross catwalks for balancing pistons on Longitudinal Plan and cross catwalk for main valves on cross section. Sorry for that.