Yes, normally, each drive attached to a port gets treated as a separate drive. Here's a rundown of some alternatives:
RAID0: Used to boost disk performance. You're right in that it combines drive size, up to a point: 2xsmallest drive. So you can get 256GB out of 2 128GB drives. But, if you went with, say, one 64GB and one 256Gb, you'd only get a 128GB combined drive. RAID0 technically isn't a form of RAID in that there is no redundancy; in theory, the odds of a drive failure are twice as high versus a single drive because you're using two disks, and, if one fails, you lose EVERYTHING.
JBOD (span): Combines 2 or more drives into a single volume. A 128GB SSD and a 256GB SSD, for example, can be combined into a 384GB drive. No performance boost, but if one drive quits, you only lose what's on the failed drive. I didn't scour those laptop docs so I don't know if it supports JBOD or not - that's probably buried somewhere in the user manual or something.
RAID1: Runs two drives in parallel. All data is written to both simultaneously so that, if one fails, the other will still run. Used for redundancy and availability. You can still use a system with a failed drive pretty much like normal. If you replace the failed drive, the system will run more slowly while the new drive is rebuilt, but it'll still function. A bonus of RAID1 is that you can, in theory, get much better read performance, which nearly all of what Trainz and many other apps do while running, although, in practice, most consumer-grade RAID1 setups don't benefit much. One thing to keep in mind with RAID1 though, versus a traditional backup, is that it doesn't protect you from file deletions, malware that corrupts your files, and the like. It's great for people who need the availability or who just can't back up as often as they should, but it's not really a substitute for proper backing up.
By the way, "memory" is not the same thing as "storage": Simply put, memory is the temporary memory used while your computer is running whereas storage is the long-term storage for programs, files, and everything else, whether or not the computer is running. Memory is almost always much more limited than storage and is temporary, but is usually faster.
As for whether it's "ridiculous" to exchange a 1TB hard disk for a smaller SSD, well, to some people, the extra performance is well worth the reduced capacity. I love my SSD. I'm still not completely sold that SSDs are reliable enough to fully replace HDDs at this point, and supporters of each can have a blast debating the merits of each, but there is no need to debate here. If you want a hard disk, get one, and have an SSD or two for the stuff that needs performance.
Here's a possibility based on what works for me: I have one giant Trainz installation with something like 600-700GB of content. I have a smaller, "driving" installation stored on my 128GB SSD which has just a few routes I like to drive. I occasionally copy over routes from my big installation to my driving one and delete any old ones (and dependencies) as necessary, apart from a few favorites which I refuse to delete. It's worked well for the last several years, although between my SSD filling up with FSX stuff and the prices of SSD storage coming down to more sane levels, an upgrade might be in the near future. Keeping this example in mind, it's perfectly reasonable to have an HDD as an OS and, perhaps, as a "master storage", drive and one (or more) SSDs for the stuff that really might benefit from it.
Lastly, to kind of sum things up, it's probably best to go with a non-RAID setup, have a 256GB (or thereabouts) SSD for the boot drive (I'd pick a 256GB Samsung 840 or Intel, depending on your budget) and for running Trainz, and a 1TB hard disk drive for major storage. Unfortunately, it looks like they don't give you the option of making the HDD a boot drive, and this might violate your plan of having Trainz and the OS on two separate drives; if that is really important, perhaps go with a second SSD in slot 2 and see if it can be spanned/JBODed together with your main SSD. Alternatively, maybe go for no JBOD, get a 128GB SSD for the boot drive, and a 256GB (or whatever you can afford) for the slot 2 drive, and a hard disk. It kind of hurts to recommend a 128GB though since they're so close in price to a 256GB drive, though.