I have located the nature of the problem, it the weight! i added, as suggested, 1 kg of load (those used in gym, i don't know their name) and the train parted! Now, another problem, how do i hide a kg in the tender, under the wood?
Also, a couple of days back, i cleaned eveything, oiled the trackes, i believe these are fine. i found under the tender a circular/rubber made thing, like a wreath/crown. i checked under the tender, but there is not anything missing. any ideas?
Thank you both for your replies, much appreciated!