Well if there's a choice here, unquestionably the desktop is better, as with pretty much anything that requires serious power. I'm not much of a laptop guy and some folks here have gotten it to run on them, but usually with fairly modern/fast hardware. But unless mobility is an absolute requirement, yes, a desktop.
I ran until pretty recently a minimum-spec system and TS2010 ran smooth as silk:
CPU: E2200
RAM: 1Gb DDR2
Video: nVidia 8400GS
OS: Windows XP Pro 32-bit
Mobo: Biostar T-Force 945P
Personally I'd say desktop, easier to upgrade, internally clean and work on in general.
If you get a laptop make sure you get a very good one or you'll be wasting your money.
Minimum specs for 2010, XP SP3, dual core CPU (the higher the better), n/vid 8000+ (again get the best you can) or ATI equivalent, 4 gb RAM. PSU ?? make sure it's powerful enough to supply the system with some to spare.
My system which didn't cost the earth runs trainz well.
E8500 core duo 3'16 ghz, N/Vid 9600 GSO, MSI mobo (P45), 750 watt PSU, 4 GB ram.
Only get a laptop if you want to work with Trainz outside your home base (or are short of space for a desktop). In either case you must get one with a dedicated graphics "card".
A desktop will always give you a "better bang for your buck(s)".
As a general view in PC buying get the best you can afford (without breaking the bank).
You should get at least 3gb ram (more would be better especially if you get Windows 7 64 bit as the OS) and at least a 2 core CPU. For desktops, make sure that the system can be upgraded in the future. Be aware that many "off the shelf" system from otherwise good manufacturers (e.g. Dell) have low rated Power Supply Units (PSUs) - around 350w. These will not power high end graphics cards if you want to upgrade in the future and the PSUs can be difficult to change.
Minimum and recommended spec for TS2010 can be found here (bottom of the page):
Here's what i'm running Trainz on now. Asus G50VT,intel centrino 2 duo core P8400,2.26GHz,4GB memory,64 bit,vista home premium,NVIDIA 9800M GS,512mb Dedicated.
That sounds like what I need to replace my 3 year old Dell Inspiron, which chokes on TS2009/TS2010 as soon as you start to paint terrain textures. In the context of the OP, then, if you're going down the laptop road you really are going to need a high level machine if you're looking at doing developing.
