The following scenario might help make things clearer. Note that all prices are made up, because I can't be bothered to look anything up.
I want to go from London to Manchester today*. I go to Avanti's website (or any other TOC or 3rd party retailer), the through fare is £100. Great, I can buy this online, or go to a station and buy it from a ticket office or machine (or even on the train from the guard). It will cost me £100. No commission or booking fee or anything*. I can even put the journey into TrainSplit, specify "no splitting", and this fare will come up.
However, because I know a bit about railway ticketing, I know that I might be able to save money by splitting the ticket. So before I book, I put the same journey into TrainSplit. Wow! I can get the same train for £66! What a bargain. This saves me £34 compared to buying from a TOC (be it online or in person). So TrainSplit is cheaper.
Further examination reveals that TrainSplit is selling me a London to Crewe ticket and Crewe to Manchester ticket. These cost £50 and £10, respectively, so £60 in total. But wait a minute! Why is TrainSplit charging me £66? Well, as their website says, "If we find you a saving using split ticketing, we take a commission of up to 15%. If there's no saving, there's no fee". So, the £60 ticket cost saves £40 compared to the through ticket. 15% of £40 is £6, hence the TrainSplit price of £66.
I'm a cheapskate, and I don't want to pay the £6 commission TrainSplit wants (even though I'm saving £34 already), so I'm going to buy the two tickets separately. I don't mind carrying extra bits of paper/having multiple PDF documents. To do so, I can:
- Book two separate journeys (London to Crewe, Crewe to Manchester) on TrainSplit. Because the system isn't generating any splits for me, I don't pay any commission. Thus the total cost is £60.
- Do the above, but using a TOC or other 3rd party website. Again, this will cost me £60.
- Go to a station, and buy the tickets separately from a ticket machine. Again, this will cost me £60 (but I'll only be able to do this if the machine allows me to change the origin).
- Ask for "London to Manchester, split at Crewe" from the ticket office. If the seller agrees, and can find the correct fares, this will also cost me £60.
As can be seen, all four options here cost the same. Therefore, there is no disbenefit in buying the tickets from TrainSplit, which then benefits from the "retailer's commission**" from the tickets which helps to keep the splitting service going.
Hopefully this is a nice, clear example.
I'm happy to be corrected if I've made any errors in this post.
*Because I'm travelling today, Trainline won't charge a booking fee.
**Not to be confused with split saving commission, or booking fees.