My current collection

That's one of the two coaches from the 2002 S-29 Plakids C12 Steam Locomotive. I have three complete sets, one is Made in Thailand, the other two are Made in China. I have also just now found a picture of one marked Vietnam.  The coaches do not have TOMY written on them. The open-able roofs have the same clips as yours, so I'm a bit mystified as to why you can't get yours open.