Large Transport; Low Wing; 2 Turbojet Eng
Flight Crew Last 90 Days 153; Flight Crew Total 10049; Flight Crew Type 5022
Human-Machine Interface; Confusion
Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Deviation / Discrepancy - Procedural FAR; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Issued Advisory / Alert; Flight Crew Became Reoriented; Flight Crew FLC complied w / Automation / Advisory; Flight Crew Returned To Clearance
Airspace Structure; Procedure
Depart BOS.  Clearance was HYLND4 HYLND MERTO YSO.... As filed.  First officer loaded the route and I checked it.  He inadvertently entered METRO into the FMC.  Since MERTO and METRO are almost identical; I too missed the error.  As it turns out; MERTO and METRO are similar in distance from HYLND.  As we passed over HYLND; the FMC started a left turn toward METRO.  The controller immediately recognized the left turn and instructed us to go to MERTO.  Even as she gave us the phonetic spelling of MERTO; it still took us three times to recognize the transposition of the R and the T.  Of course this is compounded by the relatively similar distances of MERTO and METRO from HYLND.
Proper SOP was utilized in programming the FMC with one pilot typing in the route; and the other verifying.
A flight crew was cleared from BOS via HYLND4 HYLND MERTO YSO but entered HYLND4 HYLND METRO YSO which caused a track deviation after HYLND. The error was very difficult to detect on the FMC even though Company SOP verification was completed during preflight.