How do wheels of a Train engine get grip to pull such a heavy train?

True, they make up for the low friction with lots of weight on the driving wheels. The engineer also aplies power to the wheels very gradually so as not to break traction because static friction is greater than sliding friction. Also there is slack in the soupling between the cars, so the driver sometimes backs up to close the gap, then as the engine moves forward, it only has to start one car then anotheras the slack is taken up one couplng at a time. After several cars slack is taken up, the moving cars have enough momentum to start the rest. That is why sometimes when a train starts it makes a lot of crashing noise. Also it the driving wheels break traction, they have to slow down then apply power again.

Rango has said it already. Massive weight pressing down through a tiny contact patch gives excellent adhesion that is actually quite hard to break on a clean, dry rail. The low rolling resistance that steel wheels on a steel rail offers also helps greatly. Even an averagely fit adult can push an unbraked rail vehicle of 20 tons or more on a level grade. And this is the key to how a locomotive can haul massively heavy trains.

For one thing, they are very heavy. They used to have a system that spread sand in front of the wheels for traction. I'm not sure if this is still used or not.

The trains have a device to adds sand to increase friction when needed to increase or slow down speed of train.

Rango gave you the correct answer here. id like to add that its 'steel on steel' not iron on iron. steal is made from iron and other additives. ( the iron horse is a misnomer nowadays ) while wheel slippage is OH SO detrimental to trains, it does happen. if your fascinated by this, check out youtube. search for C&O's 2716 (posing as a Southern loco [nee SR 2716]). theres a video showing her working a hard grade (with working booster!!!!! amazing!!!!) with a heavy train. you wanna hear and see a battle for gription, check her out! also a vid exists of classmate NKP #765 slipping, with no train at all.... just bad throttle handling. and ofcourse, even N&Ws 611 has been known to loose her sure footing! all these vids are on youtube. so even with half a million pounds on the rail, your summation is right, slipping still happens. takes a steady hand, experience, and sometimes a computer controlled loco to get the job done. but not always.... they did it for 150 years before computers were added for wheel slip. to play devils advocate here, next time your railfanning and you catch a train ready to depart.... watch the trucks (bogies for our european friends) on the locos. if your lucky enuf to get a heavy handed throttle jockey, you may see some interesting lurching going on as the locos 'dig in'. i garauntee you will be amazed at how well those steel wheels bite steel rail. ive seen suspension travel of quite a few inches during such an event on some GE units. what a show!

The iron on iron is extremely heavy. The static coefficient of friction for iron on iron is actually 1.0, higher than I expected when I looked it up. But the friction force depends on that and the weight force, and that weight is a huge force in a fully loaded train. Also, acceleration and deceleration is very gradual as to not exceed that friction force. That explain in part why a fully loaded freight train takes so long to stop, if they try to stop quicker they could lose traction.

Cos us ingleesh ar the best enginears which is y we made steem tranes!