There are plenty of high end laptops that function as tablets nowadays. It all depends on how much you want to spend. For me, buying the Surface Pro 3 (yet to buy it, got a review version though) will kill two birds with one stone. I want a productivity machine, but I also want something that is portable and can be used quickly. The Pro 3 ticks those boxes plus I'm loving it! The Pro 3 also means I don't have to keep an extra tablet alongside my usual Macbook as it functions as both.
It really depends on how much you want to spend, there are laptops that would play Train Sim for around £600-£700 but there are lapblets (laptop+tablet) that are around £800+ and would also play Train Sim.
You should keep in mind that Laptops aren't officially supported too, but then again loads of people use Train Sim on a Laptop.
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It would probably overheat very quickly, and intergrated graphics are not the best for heavy gaming.
Also how would you use controls: bluetooth keyboard/mouse?
Battery use would also be a problem (it will go very quick!)
Integrated graphics chips have come along way
My main desktop could play Train Sim, albeit on low, without an external GPU.