It depends on your budget. If you want to spend the bare minimum, you are pretty much relegated to the likes of HP, Dell, etc. If you are willing to go up into the mid-range, CyberPowerPC or ibuypower are decent. They tend to let you customize a little more than mainstream and also have high-end laptops as well.
Another suggestion if money is tight is bang for the buck. Be willing to settle for a smaller screen. You typically can get a little more processing power or specs in a laptop in the 13-inch to 15-inch screen range than a 17-inch screen laptop at the same price point. Since you can hook up the laptop to a big HDTV screen through HDMI, you only need the laptop screen for on the go simming. If you do hook it up an HDTV, choose a resolution around a 720p. 1080p will kill your framerates on anything but the higher-end gaming laptops.
In terms of processors, it comes down to AMD and Intel.
If you go Intel, avoid celeron/pentium based laptops. Get a minimum of an i3, but get an i5 if you can afford it. The higher the overall clock speed the better. Avoid getting an i7 based laptop because the performance increase over an i5 isn't worth the price premium. Only get an i7 based laptop if you plan on going all out and price isn't a concern. Get a dedicated GPU card if the laptop allows it, otherwise the built-in GPU should get you by. The newer top-of-the-line i7 chips are the first intel chips with GPUs that have discreet dedicated GPU card performance built-in.
If you go with an AMD chip, again... try and get a high clock speed. The amount of CPU cores doesn't really matter because TS2013 isn't really gonna take advantage of them. OpenRails is the only rail sim that I know of that WILL take advantage of multi-cores at this time. The higher-end AMD chips (the A10 series) do have the best GPUs built-in with the most GPU cores.
The advantage AMD has over Intel is that they allow the potential for crossfire multi-GPU support if the laptop allows you to ALSO instal an AMD discreet GPU card (my current laptop is set up in crossfire config). Intels can NOT crossfire or SLI with a GPU card. This allows the AMD APU's built-in GPU to work in tandem with the discreet card's GPU. This won't work with an Nvidia discreet GPU card., only an AMD card. This is only of use in games that support it, although SOME can be forced to. Crossfire WILL NOT WORK in TS2013 at this time and likely never will in the future. It will default to the better GPU unless you force it to use the built-in GPU.
Basically... Intel chips are better in the CPU department, the AMDs are better in the GPU department.
As for RAM? 4GBs will be fine, 6GB at most. For gaming in general, no more than 8GB is needed. Only HD video editing software sees advantages with more than 8GBs installed for buffering purposes.
Hard Drive? Most are fine, but SSD drives help a bit in loading.
That covers most of the essentials.
