The solution would be that you could blend one brand of toys with the other, for example: Lego with Playmobil. Toy manufacturers are clever enough to not let this happen: a Playmobil hand can not hold anything made of Lego.
My solution: ADAPTISE!
Design toy-adaptors making it possible to use one brand with the other. Our Lego collection is not that large and it doesn't contain any train yet. Instead of buying more Lego, I made an adaptor.
I took a Brio train wagon and glued a standard Lego plate on top. This wagon turned into my son's favourite train wagon. Each time I make a new (Lego) top he is so happy. Besides trains, he now can drive around with aeroplanes, spaceships, animals, etc.
Now its a matter of keep on going making more toy-adaptors. If anybody feels inspired and wants to share thoughts, don't hesitate and comment on this post.