The Qualities of TOMY/Plarail trains.

if they are looked after, they can last generations. ive had my first engine for almost 25 years, and its still going strong, after a repair a few years ago [new motor needed and lube, thats it]. i could easily pass my trains on to my nephews, but they dont look after things properly, so its rare they even see my trains, never mind use them.
like anything, if you treat them right theres no limit. smack it with a hammer and yeh, its going to break, but so will most things.

i know you said your mum wants you to get into n scale... thats not a good idea. as T-R-Y said, proper scale locos and rolling stock are expensive and very delicate. they are also easier to have issues with general running. if you dont keep the tracks clean, it gets into the motors and then you have a possibly expensive repair or worse.
another thing to consider, do you or anyone in your family have the ability to build and wire an n scale track? then youve got the costs of scenery [scale people, animals, flock, houses, trees, etc etc] next to no ready to go tunnels and elevations like trackmaster.
also, once the track is down, nailed, soldered and ballasted, theres no changing it without ripping it all up and buying more track.

you wouldnt be able to cart the locos around to sports events and where ever else you take trackmasters. when i transport mine, its wrapped in bubble wrap and in a box, then put in a bag which i wear. mine have to go on my motorbike, so wearing the bag lessens any bumps, preventing damage. and even doing this, ive had couplers broken several times. not major, but they cost to replace.