Good day to you,
To reach the reciprocating engine room from the boat deck the engineers he would have to take the reciprocating engine casing down to the starting platforms on the tank top. The casing was filled with catwalks and multiple stairs. An engineers would then have to go aft to reach the low pressure turbine engine room since the casing of the turbine engine room wasn't accessible from the boat deck.
That would depend on where the engineer would start his shift. If he would wake up and go on duty from the engineer's quarters on
F-deck he would have had accessed the reciprocating engine room through either one of the doors on the port or starboard side, depending on which side he would have had his cabin. If he would have had supper first in the engineers mess on
E-deck he would have accessed either the reciprocating or turbine engine room through the double doors which were connected to the long working alleyway, also known as Scotland road.
I would highly recommend to look at the general arrangement plans of the Titanic in case it is a bit hard to understand. It might gives you a clearer picture.
Yours sincerely,
Thomas
