How to take the rail journey from Bruxelles/Brussels to The Hague/Den Haag

If you want to travel from Bruxelles/Brussels to Den Haag/The Hague by train as quickly as possible, you can save a minimum of 40 mins by taking this combination of trains.

Though this speed advantage is somewhat negated if the starting point of your trip is conveniently located to either Bruxelles-Central or Bruxelles-Nord stations.
That's because the Eurostar formerly Thalys trains pass through those other two stations on route to The Netherlands.

The Connection in Rotterdam

Because seats cannot be reserved on Dutch trains tickets will be valid on any train from Rotterdam on to Den Haag/The Hague.
The ticket agents will typically sell end-to-end journeys which have a time between trains in Rotterdam of around 15 mins - and involve taking an IC train on to Den Haag.
Though within 4 minutes of the scheduled arrival into Rotterdam there will often be a 'Sprinter' train departing for Den Haag, so if you can make this connection on time, you can hop on board.
Though the 'Sprinter' trains are 2nd class only - and the IC trains have both 1st and 2nd class.

Conversely in the unlikely event of the train from Bruxelles arriving more than 15 mins late into Rotterdam, tickets will be valid on any train on any subsequent train on to Den Haag - there are up to five trains per from Rotterdam to Den Haag.

All of platforms / tracks in Rotterdam Centraal can be accessed by using lifts / elevators.

Book early and save: Yes

Online bookings usually open: up to 4 months ahead of the travel date

This option is faster, but it's more expensive than taking the combination of IC trains (Option 2) - and users of Eurail and InterRail passes can save more €20, by not taking the Eurostar formerly Thalys trains.

Tickets for this connection which includes a Thalys train from Bruxelles are only available from 4 months ahead of the travel date - but the alternative journey options involving an Intercity Brussels train can be booked from 6 months ahead

However, note that these Thalys trains only depart from Bruxelles-Midi/South station, but the Intercity Brussels trains also depart from Bruxelles-Centraal and Bruxelles-Nord.

There are typically two good reasons for taking this combination of two InterCity trains:

(1) you can travel between the city centres, this is the only combination of trains on which it's possible to travel in the one direction and depart from Bruxelles-Central and arrive at Den Haag Centraal.

(2) if you will be booking tickets after the cheapest deals on the Thalys trains have sold out, or will be using rail passes, this combination of trains will save money.

Though what is an odd feature is that NS, the Dutch rail operator has deliberately arranged the timetable to allow for an easy connection at Breda station, but because the time between trains is only 5 mins - the ticket agents seemingly won't place tickets on sale for this combination of trains.
The train on to Den Haag arrives in and departs from Breda while the train from Bruxelles spends around 10 mins at the station.
Five minutes should be enough time to make the connection, Breda station has recently been modernised and each set of platforms can be accessed by lifts and escalators.
So if the train from Bruxelles arrives in Breda on time, rail pass users should aim to make this connection.
Howeverm if the train from Bruxelles is delayed, remain on the train until it arrives in Rotterdam as here are more trains per hour from Rotterdam to Den Haag Central than from Breda.

Book early and save: Yes

Online bookings open: 6 months ahead of the travel date

Booking end-to-end tickets for this journey on a combination of two x IC trains should be straightforward.
Seat reservations aren't required, because they are not available on either of the trains, so there isn't a need to take specific trains when making a connection.
But despite this the ticket agents tend to only offer journeys with two connections instead of one