For any diesel traction: Sierpc and Tłuszcz is where most diesel trains start/terminate/pass. Trains are allocated interchangeably, i.e. any type can appear on any service for diesel operated traction.
https://portalpasazera.pl/en/ will say "railbus" for diesel worked trains operated by Koleje Mazowieckie - to check, click "Find a train" at the top of the page, then from the dropdown select "at the station" and then enter the station name (e.g. Sierpc). Select a train, and click on "Connection details" at the top of the page. It should then show you information about on board facilities (and, if it is a railbus operated service, will say "railbus").
It is not possible to get the specific train type allocated. Beware that the translations on the site are very poor. You can only check departures from a given station on the day of departure only (unless you have a route, in which case you can use the journey planner on the homepage and click on "Connection details" once you have selected an itinerary).
Double deckers normally operate on
some RE1, RE2, RE3 and RE8 services. They can also appear on other (electrified) routes at random. To the best of my knowledge,
Portal Pasazera does not indicate whether a train is operated by a double decker set.