They're called EZT.
In Polish EZT for Elektryczny Zespół Trakcyjny. Which has an English equivalent: EMU - Electric Multiple Unit.
I always wondered why Poland ran what were very clearly city commuter trains on inter-city runs of a few hours!
It depends on what you exactly mean.
Until a few years ago the main companies that operated the trains in Poland were PKP Przewozy Regionalne and PKP Intercity, both national ones, belonging to a holding called PKP (Polskie Koleje Państwowe - Polish National Railways). The division was that PKP Przewozy Regionalne operated the city commuter trains (so called "osobowe") and also most of the inter-city ones (so called "pospieszne"). Except for the most comfortable and fastest ones, so called "ekspresowe" and "InterCity", operated by the PKP Intercity. It was convenient, because one could buy one ticket for a whole train journey and it didn't matter how many times one changed a train and between which categories.
Then the government decided to pass all the local commuter trains to the local governments of all the provinces (województwos). To do this, they moved all the "pospieszne" trains from PKP Przewozy Regionalne to PKP Intercity (which will be important later on, also these from £ódź to Warsaw - it is rather a commuter route in terms of that most of passengers commute to the workplace with these trains every day, but it's also quite long). PKP Intercity had created a short time before a new brand of trains called TLK - "Tanie Linie Kolejowe" ("Cheap Railways"), wanting to compete with the "pospieszne" of PKP Przewozy Regionalne. It looks strange that one national company wanted to compete with another one, but it's just Polish government, sometimes it is impossible to understand it. So after this movement they changed all the "pospieszne" into TLKs (later on, probably together with an increase of ticket prices, they changed their name into "Twoje Linie Kolejowe" - "Your Railways"). And it's how the current TLK train category emerged.
With the "pospieszne" trains, PKP Intercity took over also all the carriages and locomotives needed to operate them. Przewozy Regionalne (they left the PKP holding, and they no loger have anything in common with the PKP, apart from that they are running on tracks managed by another company from the PKP holding, and the mentality of the employees is similar, especially the trade unions are extremely influential) stayed mostly with old-type electric multiple units (in Polish - EZTs) of the EN57 type. Sometimes called (especially by train spotters and other train fans) "kibel" from the specific "toilet" smell inside. If someone doesn't know, "kibel" in Polish is a quite impolite (but definitely not vulgar, AFAIK it came from the jail slang) word for a toilet.
PKP Intercity started to cancel many of former "pospieszne" trains, argumenting that they are inaffordable and bring financial loses (which is quite a stupid explaination, if it brought money, the governement wouldn't have to involve in it at all - but again, noone will understand the Polish government). So Przewozy Regionalne decided to fill this niche, in case of specific trains either without any subsidies (and the trains brought money, though), or with subsidies from the local governments. They named this new category of trains "InterRegio", and the former "osobowe" were named, in contrast, "Regio". Later on they created also a new, a bit more comfortable train category (for sure you won't met there EN57 EMUs), but still an affordable one, called "RegioExpress".
Since Przewozy Regionalne have not much locomotives and carrieges (they have to hire locomotives for inter-city trains either from the Czech Railways, or from the company of the PKP holding that specializes in cargo transport), most of the InterRegio trains run on the EN57 multiple units. It's the explanation if you meant just the InterRegio trains.
If you ask why you sometimes (but rarely) can meet EZTs designed rather for commuter trains running the TLK trains, the explanation is also connected with all what I mentioned a moment ago. Yet in the times of PKP Przewozy Regionalne in the PKP holding, operating also most of the inter-city trains (exactly about 10 years ago), the route from £ódź to Warsaw started to be renovated. Now they are finishing these renovation works - it took so much time to them. Time, in which the trains went not faster, but yet slower and they were extremely unpunctual - delays were the order of the day. Together with this renovation, they (meaning PKP Przewozy Regionalne) purchased new trains for this route. Pesa Bydgostia multiple units - the type ED74. And they were used on this route for a few years, but they met a lot of criticism from the passengers. They were to small for this route, which is probably the most popular train in Poland (maybe apart from the city railways for example in Tricity). And too uncomfortable for such a long journey (during the renovation works it was and still is about 2 hours, after the finish it is going to be more than 1 hour). Indeed they are good rather just for real commuter trains.
In Przewozy Regionalne it wouldn't be such a problem. They could move these units to other, really commuter, routes and introduce some of the carriages they have on the Warsaw-£ódź line. But these Bydgostia units, together with all the vehicles for "pospieszne" trains, were taken over by the PKP Intercity. At some moment they had to withdraw these trains form this route due to the fact that the number of trains daily had to be reduced because of the lasting renovation works. Their capacity was to low to fit in all the passengers, taking into account that they can be linked, AFAIK, maximum in sets of 2 units. And they are one of the most modern trains PKP Intercity has, so they don't want to sell them (again stupid, but it's impossible to understand our government). This way they appear on long-distance routes although they shouldn't.
A bit of explanation about the other things I started talking about and might catch someone's interest. The names of the categories of the most comfortable trains. In the communist's times they were called "ekspresowe" only. In the 90s they introduced a new category - InterCity. It was stable during the 90s and initially after 2000, but later on they started to juggle with their names, so that at some moment for example a category "Express InterCty" appeared.
And now "Express InterCity" is the only category of this kind of trains, of course apart from "Express InterCity Premium", which are the Pendolinos.
A very new idea, which appeared together with the introduction of the Pendolinos, was to change the name of the TLK trains operated with the most modern carriages to "InterCity". It's now not a premium category, it's a standard one, like TLK. The prices are, AFAIK, the same as in case of TLKs.
Another thing. I mentioned that the local governments have in fact no control of the Przewozy Regionalne, because it's their common possesion. What is more, the trade unions are there very influential. It's why they tend to buy new trains (I mean vehicles) on their own, not by Przewozy Regionalne, and then lend them to the Przewozy Regionalne. They tend also to create their own railway companies and resign from the services of the Przewozy Regionalne, even though they still have shares in it (bear in mind that the Przewozy Regionalne bring losses, not money, so noone would like to buy their shares; to posses their shares means to have a debt). This way the following companies appeared:
- Koleje Mazowieckie (Mazovian Railways) - their are the oldest one and they weren't created as a separate company, but separated from the Przewozy Regionalne
- Arriva RP - it's not a company owned by a local government, but a government of the Kuyavian-Pomeranian Province hired a "private" company (in fact connected with the German Railways) instead of Przewozy Regionalne to operate local trains
- Koleje Śląskie (Silesian Railways)
- Koleje Dolnośląskie (Lower-Silesian Railways)
- Koleje Wielkopolskie (Greater-Poland Railways)
- £ódzka Kolej Aglomeracyjna (£ódź Urban Agglomeration Railway)
- Koleje Małopolskie (Lesser-Poland Railways) - started operation this Sunday, together with Pendolinos in PKP Intercity
There are also companies operataing urban trains within the biggest cities, and so:
- PKP Szybka Kolej Miejska w Trójmieście (PKP - Fast Urban Railway in Tricity), belonging to the PKP holding - in fact it operates also regional trains, so should rather be classified as a member of the previous group, but initially it was only for the urban agglomeration
- Szybka Kolej Miejska w Warszawie (Fast Urban Railway in Warsaw), belonging to the local government of the city of Warsaw
- Warszawska Kolej Dojazdowa (Warsaw Commuter Railway) - belonging mostly to the local govermnent of the Mazovian Province, being a reminder of the suburban light railway system from before the Second World War (an interesting thing is that a similar system in £ódź after the war was classified as trams and developed in totally different way, unfortunately in Poland doesn't exist anything like "light railway", there is a sharp distinction between trams and heavy railway)
I think, that's all.