My son loves both and my mum has bought him a thomas mega bloks set for his birthday (in 48 hours!!) and I have bought him nothing 
I know I want to buy another set to add on to this so that he has a bit more track to play with (he loves this kind of thing at toddler groups) but I don't know what to go for. We saw the Chuggington one in Tesco and he nearly peed his pants with excitement, but then rounded the corner and saw the Thomas one (that my mum already got him) and he was just as excited.
Are they compatible with one another? I don't want two different train sets, but seeing as they are both mega bloks, would it be unreasonable to assume that they would be interchangeable?
It would be nice if he had both sets of characters, but not if I have to store two different types of track.... 
Thanks in advance!