Hi folks, I'll need to do this journey in late June. I'll be travelling midweek, likely around 13:00 onwards, does anyone have any tips, e.g. is it better to book in advance? Is First Class any good? Is there a dining car where I could sit and eat / drink with my luggage for the journey? Which side of the train will give me the best views? All suggestions appreciated. Thanks.
Some good advice has been given, but some of it may be outdated with Covid rules and some of it just is bad advice/ wrong.
Seat reservations on all PKP trains (TLK/ IC/ EIC/ EIP etc.) are now compulsory. Reservations on regional trains are not. You will likely be on a PKP compulsory reservation train on that route. Midweek is June shouldn't be *that* busy so reservations on the day should be more than do-able. Whilst you can buy a ticket onboard, you have to seek out the staff onboard rather than have them find out, and even then there's an additional fee.
TLK used to be cheaper than IC, but isn't true any more. It also isn't always slower than IC so don't listen to the poster above. A lot of politics is involved in Polish railways so even an IC train can stop at a village halt. The main difference nowadays is the standard of the carriages.
I would generally strongly recommend avoiding TLK trains and seek out IC or EIC ones. The main difference is that IC/EIC are more modern, have A/C and are generally a bit more spacious than TLK trains.
First class on compartment trains is three per row with nine compartments per carriage. On open carriage trains it is 2+1.
Second class on compartment trains is three per row on EIC. It is usually three per row on IC, but can occasionally be four per row. It is usually four per row on TLK. There are generally ten compartments per carriage. It is always 2+2 on open carriage trains.
Is first class worth it? Well, other than more space, it provides no extra service. However, the extra price is pretty small and Polish trains are cheap compared to the UK. If you have a fair bit of luggage, it will be easier in my experience. However, it is totally up to you.
Dining on Polish trains is provided by WARS. You can find the menu on wars.pl. I suppose it is fairly expensive by Polish standards, however the food is very decent and is good value by British standards. I would recommend it for a proper meal at least once. The EMUs don't really have a proper dining carriage with just a fairly small dining space. However, the traditional carriage trains have a more traditional larger dining space which are pretty nice. You can find people just having a drink on most trains, but this isn't an issue at all and people are not getting rowdy or drunk.
Check
http://bocznica.eu/ or intercity.pl for whether the train has a dining car or not.
Unfortunately, the scenery between Wroclaw and Poznan is fairly flat and uninspiring. You aren't going to see any great views, but it is pleasant enough.
Whatever you do, don't buy from anyone other than PKP themselves (intercity.pl). Their website is in English and accepts foreign cards without issue. Why would you pay an agency fees when you don't need to? TLK and IC trains do not have advanced fares and you'll pay the same fare whether bought in advance or on the day. Only EIC and EIP trains, which don't operate on Wroclaw to Poznan, have advanced fares.