Track base/underlay for N gauge layout

You can fix it directly. However, it will have a tendency to "drum" a bit on the board. How much hassle this is depends on how well supported the MDF is.

You could use card, though apart from giving a bit of a "shoulder" to the balast, I don't think it will give any improvement in noise transmission. I've used thin balsa under track to form a shoulder for the balast.

The issue with quiet running is to use a flexible fixing. If you glue everything solid with PVA adhesive and granular balast, the noise will be transmitted. Flexible adhesives (eg Copydex) will be a bit quieter.

Suggestion (which I've not actually tried, so experiement on a small area): Fix the track down with double sided selotape. Scatter ballast material over the exposed sticky areas and tamp-down with a suitable stick.

I've used double sided tape for demonstration layouts, but not with any ballast.

(My active modelling is 2mm finescale, whilst the size is similar to N, the track is solder construction, so somewhat different. However, I have done the odd bit of N in the past).

Foam ballast isn't cheap but it does provide quiet running and its easy to lay, only available for Peco points but the plain track should fit any make (I think). Fleichman track is ready ballasted but expensive, the Fleichmann point motors can be turned upside down - If you make a hole in the baseboard the point motor can be covered with thin card to hide it. Tomix track is ready ballasted and has point motors built in but it is even more expensive. Ballasting track is tedious and put very little round the moving end of the points, its not only the moving blades there is also (on Peco points) a small spring that needs to be in the clear. I have used Chinchilla sand with some success, I used flexible PVA (book binding glue) but that isnt cheap, another flexible alternative (I havent tried) is artists acrilic matt medium.

HTH