The main thing when buying a PC is to check the CPU speed, expressed in GHz ... 3.4GHz being very high end ... and anything less than 2.0GHz is really, really, low end.
Some Alienware PC's have 4.2GHz CPU speed (but cost @ $5000) lol
Turbo boost speed is not relivant to Trainz nor gaming, as turbo boost is for short sporadic bursts of speed for general computing ... not for extended periods of gaming.
So the turbo boost speed of 2.7GHz is a deceptive advertising spec, as the PC may only have 2.0GHz CPU raw speed.
A 1Gb to 2Gb video card is also important
If you are buying a desktop, you only need 4Gb RAM (maximum), a 16Gb RAM PC is no better, as Trainz only uses 4Gb RAM (I would recommend 6Gb RAM plus, though).
A 1Tb Hard Drive is fantastic, but you really don't absolutely need a 1Tb HD
A Power supply should be in excess of a teeny 300Watt Light On power supply ... you can replace a 300Watt PS with a higher wattage PS, (for @ $80, plus a technicians professional install for another $80) then you can install a high end video card (for @ $80 to $250).
My 3Gb RAM desktop has a 300Watt PS, so I could only put in GT430 video card
I have read that some guys take an old XP PC and turn it into a super computer for a couple hundred bucks.
There are Big Box store Gaming desktops @ $800 (some are cheaper online)