In message , at
23:41:45 on Mon, 21 Jan 2013, Arthur Figgis
remarked:

Does Brussels have some kind of airport-weighting on the fares on the
new line?

I haven't used the train from the airport for several years. However,
Wikipedia says:

A direct train link with Leuven was opened on 12 December 2005.
A direct link with Antwerp and Mechelen via the so-called
Diabolo line was opened for public service on 10 June 2012. The
Diabolo project is a public private partnership. It has been
decided that all rail passengers to the Brussels National
Airport railway station station pay a "Diabolo supplement" to
finance the ongoing and planned work."
--
Roland Perry