My four year old loves Chuggington and received quite a few Diecast Chuggers from Santa this past Christmas.
She gets a lot of play out of them; loves impersonating the various voices, inventing her own Chuggington episodes, and incorporating them with her playmobil & lego playsets.
They are durable little trains. I like that the cars can 'click' together, and that a few have add-on train cars.
Good fun all in all (even if I do think the RRP is a lot for what they are) but thankfully it pays to shop around :)