There is no RER service of commuter style trains between the Gare Nu Nord, where the Eurostar will arrive, and the Gare Montparnasse on the other side of the city centre; which is where the trains on to western France will be departing from.
So if you don’t want to take a taxi, then the Metro is the fastest public transport option - though bus Line 91 also goes to Montparnasse from the Gare Du Nord, it leaves from the stop on Rue de Saint-Quentin
Taking the Metro looks straightforward, as Metro Line 4 provides a direct link between Gare Du Nord and Montparnasse (direction Mairie de Montrouge).
It’s also at least 5 x cheaper than taking a taxi and almost certainly faster too, despite it being a 14 stop journey; you will be on the Metro train for around 20 - 25 minutes.
The typical total transfer time, from stepping off the Eurostar at the Gare Du Nord to being on the concourse at Montparnasse will be 40-45 minutes; but queues at the ticket machine can add another 10 to 15mins
The four stages for making the transfer to metro line 4 at Gare Du Nord from Eurostar and Thalys train arrivals:
1: Walk ahead to the front of the train and exit on to the main concourse (the only exit from the platform/voies).
2: There are TWO routes froto Metro Line 4, neither of which are particularly clearly signposted.
The easiest option is to turn left when you exit from the voie (platform/track) and keep walking ahead until you are at the far side of the concourse - keeping the trains to your left.
In this part of the station you will come to an escalator that leads to a lower concourse.
The above image has been taken from the top of this escalator, when you step off of the escalator, initially walk ahead of you, but then turn sharp right, the Line 4 entrance will then be ahead of you.
Alternatively use this entrance pictured below - which is in the middle of the concourse - opposite the exits from voies 5-8.
If you use this entrance, turn to the left when you reach the foot of the escalator/stairs.
3: Buy a [t+ ticket]https://www.ratp.fr/en/titres-et-tarifs) at the counter or machine: the machines have excellent English translation, but look out for the easy to miss 'confirm your purchase' button.
Though at busy times and/or when the staffed ticket office is closed queues can build up for the ticket machines; it can take more than 10 mins to buy a ticket.
Or you can buy a single journey t+ ticket on your phone.
4: Make your way to the platform for the Line 4 trains heading to Mairie de Montrouge.
Before opting to take the Metro from Paris Nord to Paris Montparnasse, these six things are worth considering:
(2) The access to the Line 4 Metro platform at Gare Du Nord is NOT step free.
(3) There are no luggage spaces/racks on the trains and they’re not air conditioned either, so they can be uncomfortably hot!
(4) You will leave the train at Montparnasse – Bienvenüe station, look out for the stop after St-Placide.
(5) Montparnasse – Bienvenüe metro station is connected to Paris Montparnasse station by long passage ways.
So long that despite the provision of a travellator, it takes around 12 minutes to from stepping off the Metro being on the main concourse at Montparnasse (a long way to run if you're in a hurry!)
You will first have to negotiate three short flights of stairs to access the travellator.
And the travellator itself is long!
(6) When you finally exit the metro station, the mainline trains will be two levels above you, though there are banks of escalators and lifts/elevators.
If you will be taking a TGV from Paris Montparnasse - from the Metro, follow the signs to ‘Lignes Grande Vitesse’.
The first level above street level isn't where the TGVs depart from. you will need to go up another level into Departure Hall 1, pictured below.
From here walk ahead, under the departure board, to access the voies (platforms/tracks) on the main concourse, that the TGVs will be departing from.
If your train hasn't arrived in the Gare Du Nord exceptionally late, you won’t be able to automatically swap train departure specific tickets to a subsequent departure from Montparnasse, IF you miss your booked connection due to a delay on the Metro, OR because your taxi got stuck in traffic.
So look carefully at the combination of trains that can be offered by ticket agents, our preference is to allow a minimum of 75 mins between the arrival time of a train at Gare Du Nord and the departure of a train from Gare Montparnasse.
I wanted to share my passion for train travel and explain how anyone can take the fantastic journeys I have taken.
This is one of more than 100 train travel guides available on ShowMeTheJourney, which will make it easier to take the train journeys you want or need to make. As always, all images were captured on trips taken by ShowMeTheJourney.