Is it comfortable to get from Barcelona to Pyrenees (La Molina) for one day skiing trip in January? By train / bus / car / blablacar? I mean train would be probably the best but is there any? In January roads can be not in the best condition so rented car is rather last option. I will not have my own skiis on the trip (I'll rent them on the spot).
La Molina's own website says that there are train and bus options.
The bus is provided by Sagalés and runs once daily, departing Barcelona at 06:30 and (if I understand correctly) departing La Molina at 16:00. I'm not sure how much time that would give you on the slopes. Note that it is dependent on there being enough interest on a given day.
The train is Renfe Rodalies (middle-distance), and would probably allow you more time on the slopes because the first departure from Barcelona is at 06:08 and the last departure from La Molina is at 19:24. What isn't clear is how to purchase the package which includes the train ticket and the ski pass; perhaps it's best done in person on the previous day.