I know there are trains between the two but does anyone know if there a direct bus route or do you need to change buses somewhere? I can't seem to find the information online (at least not in English). I normally check this site for bus routes http://www.thailandee.com/eng/transportation-thailand/travel-by-bus-thailand.php but don't find any listed between these two towns.

Thanks. In that case it might just be easier to take the train

There most likely is a 2nd bus route that does BKK -Korat via Ayuttahya & Saraburi (one of the old routes) , just call Korat Bus station and ask them - this is always the easiest way to find out specific or obscure bus routes.

(I'd suggest that there probably is a mini van route for factory workers which does much the same via Saraburi and which will be quicker than a bus)

As a rule though one would always want to avoid a 2nd class bus given there stop almost anywhere and everywhere. For this journey, even if the bus/van option exists the train trip is of course much nicer and more relaxing.