rsh913

Pickles

20 minutes should be plenty if you know where you are going, don't dawdle, and don't stop to buy crap at all the stores that will be on your path. Get out of the Shinkansen side into the JR side through the JR transfer gate (same ticket) and head towards the Marunouchi (West) side, North or Central exits.

Find the escalators or elevators that go down into the bowels of the Yokosuka-Sobu line. The NEX usually leaves from track 3 or 4 of the Yokosuka-Sobu line, at level B4F. It's all well-signposted, although you'll probably be doing 5-6 floors of vertical down from the Shinkansen tracks, which are outside and on the Yaesu (East) side of the station to the NEX tracks, which are on level B4F of the Marunouchi (West) side of the station.

You could probably run the gantlet in 5 minutes without luggage and without dawdling. Add time for the 4 kids + luggage, and 20 minutes should be fine, but no dawdling or bio breaks!

abmj-jr

I would suggest taking the same shinkansen only as far as Shinagawa station and connecting to the same Narita Express there. Arrive at Shinagawa at 10:03, depart at 10:23. You'd have the same 20 minutes for the change but at a MUCH simpler and easier station to navigate. The Narita Express is the same one you would be catching at Tokyo. You'd just be getting on one station earlier. It then goes through Tokyo on the way to NRT.

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Pickles

Absolutely, this works if your NEX is the one coming from points West (e.g. Shinjuku) or South (e.g. Yokohama). If this is the case, this is what the OP should do. Fewer up/down, shorter walk, easier to navigate.

Your Shinkansen ticket will let you get out at Shinagawa, no problem. Only "snag" is that if your NEX tickets originate and Tokyo, you're in theory liable for the express surcharge between Shinagawa and Tokyo, but the chances of the conductor asking for tickets in the segment are low. If you do get caught, you can pay the surcharge on the train in cash or using a SUICA. If you haven't bought your NEX tickets, you should buy them to originate in Shinagawa.

rsh913

mnbp

The N'EX is a rather complex service. Some trains operate as a single through service, while other trains have two origin points and then meet at Tokyo station for the continuing journey as a longer train to Narita airport.

JR-EAST Train Reservation

This is the timetable for the Narita Express service. Look up your train departure from Tokyo station and see if there are two trains that depart Tokyo station at the exact same time with the same train number.

Example: Narita Express train number 33 has two parts. One part of this train departs Yokohama at 13:59, stopping at Shinagawa at 14:20. The other half of this same train departs Shinjuku at 14:10. At Tokyo station, the two parts of this train connect together, and the train continues to Narita airport. By the way, it is not possible to walk from one half of the train to the other half - the "engine" is in the middle once the two separate trains are joined at Tokyo station.

The problem obviously is that Narita Express is a reserved train. You have a seat departing Tokyo station, but it MIGHT be a seat that is on the half of the train that originates at Shinjuku (not stopping at Shinagawa) and not the half of the train which originates Yokohama (which does stop at Shinagawa).

Please note. I agree completely - Tokyo station is a bad place to change trains from Shinkansen to Narita Express (it's a long walk from the Shinkansen platforms above ground to the N'EX platforms in one of the sub-basements of Tokyo station). Shinagawa is a much better place to change from Shinkansen to N'EX.

Be sure to look up your train in the timetable and see if you might have lucked out and be booked on one of the N'EX trains which don't split/join at Tokyo station.

EDIT: N'EX train number 17 is one of the few that does not split. It departs Shinagawa at 10:23 and Tokyo at 10:33. Looks like OP will be just fine getting on at Shinagawa!

Pickles

If that's the case, you can stand on the train for the 8 minute ride from Shinagawa to Tokyo. And then at Tokyo get off the train and get back on the proper half of the sausage. Or you can stay in the "wrong" half and then find an open seat between Tokyo and Narita. If you have a reserved seat ticket and are in the wrong seat, but nobody has booked the seat, the conductors don't mind.

rsh913