Inspired by this look at the busiest routes from Vienna airport and night train routes comparison, I thought I would look at the same for Brussels https://twitter.com/sleeping_train/status/1352974400195342336

Let's go back to late a 1977 Cooks. There are plenty of night trains leaving Brussels every evening.
Dark blue routes run daily
Deep blue routes are seasonal, often run just once weekly
Grey routes are car-carrying trains

The maps were made using http://gcmap.com 

At Brussels-Midi, you would find
Riviera Express Roma/Genova
Oostende Wien Express Wien Westbf,
Ost West Express Moskva
Flandres Riviera Ventimiglia
Night Ferry London Vic.
Tauern Express Salzburg + København
299 Milano, Brig via Bern + Chur via Zrh

Many of the above starting from Oostende for London connections. Leaving in amongst these you would have car-sleeper trains from Schaerbeek to Narbonne, Ljubljana, Biarritz, Milano and Auray. Peak extra portions to alpine destinations too such as Bolzano in the 'Ski Express'

11 years later, 1988, similar network, minus the Night Ferry. There are more ski night trains to French alpine destinations (Latour de C, Pau, St-Gervais, Bourg-St-Maurice), a portion to Brig via Lausanne,the Freccia del Sole to Ancona. The Genova portion extends to Ventimiglia

8 years later in 1996, a few cuts. No more motorail to Ljubljana, or Innsbruck via Buchs. The Camino Azul runs often to Port Bou for Barcelona. The Roma portion is reduced to weekly.

A few summers later in 2000, more noticeable cuts. No more portion for København, Klagenfurt or direct Moskva cars. The night trains to southern France are less regular too. No more motorail from Brussels, all moved to Liège. The network is not as healthy as it once was.

This map is a bit of a joke. By 2006, the next timetable I have, the whole network is gone. No night trains from Brussels to anywhere. This is five years after Ryanair in 2001 first opened a base at "Brussels South Charleroi". This is the era of €1 fares and low-cost expansion.

Fast forward to 2020, night trains run from Brussels once more thanks to ÖBB Nightjet. There is a twice weekly train with portions for Innsbruck and Wien.

Projecting ourselves forward to 2024, there is a planned small network emerging of ÖBB Nightjet to Wien and Berlin and a yet-to-be awarded Brussels-Malmö service. This is a good start, but where next?

But where is there demand for travel? We can look at Brussels Airport figures for top 30 Europe destinations.
is >750k pax per annum
is >500k
is >350k
is >250k
The rings are ranges of 500 and 1500km from Brussels, the night train sweetspot.

A huge chunk of traffic is within 1500km. We can see that there is high demand to Mediterranean destinations:
Spain, a Brussels-Barcelona(-Alicante?) sleeper would fit this well. Ideally it could be adjusted for high summer demand. A first step before a direct Madrid train.

Italy too, a mainstay of the night train network in the past. Milano (-Venezia?) is a key city in range and must be a priority to return. Rome would be good too but perhaps this is too far, or have Swiss base tunnels brought it within reach?

France, a key route to reinstate is a Nice-Ventimiglia train as this serves a string of high-demand destinations in Côte-d'Azur, such as Marseille, Toulon, Antibes, Cannes and Monaco. In this case night trains can bring you to the doorstep and have a clear advantage over

What the flight data doesn't tell you is people who will drive by for their holidays abroad. These people - often families who might travel in a couchette - too are a key market for night trains.

So, what do you think key future night train routes will be?