Hello! Don't know if this is the same in different currencies, but in PLN the price is different on Steam and Epic. For standard version it's 179,99 on Steam and 169,00 on Epic. For deluxe version after discount it's 193,49 on Steam and 188,10 on Epic.
i found that the epic store is usually a little cheaper to buy the main game... as they are trying to win you over from steam, But then once they have got you locked into buying all further DLC's from them, you don't get the DLC as cheaply as it is available on the steam store... there's a steam sale for TSW content on almost all the time, so you often need to wait at the most a month for the DLC you are interested in to have it's turn in the steam sales again.
Not to mention that the pricing for ALL loco DLCs on Epic Game Store was increased from €13.99 to €17.99 shortly before the release of Rivet's loco/updated mini-route DLC IoW/Br class 484 (for no convincing reason at all), not so on Steam.
For a little bit of context here - there are a few 'price points' we can use with the various Store providers (Epic, Sony, Valve, etc.), so whilst we try to ensure parity across products, sometimes for different currencies across different platforms there can be some slight differences. Hope this helps to clear things up a bit.
Doesn't explain why one and the same old loco DLC can be sold for €13.99 in June but now costs €17.99 in August.