1. on purpose, deliberately, specifically, specially 

Expres A common synonym for 'expres' is 'met opzet'. You can use both, apart from the translation of 'specially', see the examples below.

'Expres' can also refer to an 'express train', see the second translation below.

– "Wel! Je deed het wel expres, ik ga het tegen mijn moeder zeggen!" 
("You did! You did do it on purpose, I'm going to tell my mother!")

– "Ik maak expres af en toe een fout om te checken of mensen wel opletten." 
("I delibaretely make a mistake now and then in order to verify that people are paying attention.")

– "Die gast blijft gewoon expres op de linkerbaan rijden zodat ik niet kan inhalen!" 
("That guy keeps driving in the left lane on purpose so that I can't overtake!")

– "Ik kom expres vroeg in verband met de files en nu ben jij onze afspraak vergeten!" – "Tss, denk je dat ik expres voor jou mijn bed uit kom?" 
("I specifically come early to avoid traffic and now you have forgotten about our meeting!" -"Pff, do you think I get out of bed specially for you?" Note 'in verband met': in connection with, with reference to. Usually the connection is implied, in this case to avoid traffic jams.)

– "Met opzet": on purpose, deliberately.
– "Iets met opzet doen": to do something on purpose.

– "Het spijt me, echt! Ik heb het niet met opzet gedaan!"
("I'm sorry, honestly! I did not do it on purpose!")

2. express train 
[de ex-pres]

In this translation 'expres' is not commonly used in the Netherlands, maybe in the case of an international train, although 'internationale trein' or more specifically 'de Thalys' would be more common. However when on holiday we may use it to distinguish regular trains from express trains.

– "Pakken we de gewone trein of nemen we de expres?" – "Dat maakt me niet uit, ik heb geen haast, jij wel?" 
("Shall we take the regular train or the express?"  - "I don't care, I'm not in a hurry, are you?")

Related words:
– Stoptrein: slow/local train [noun] [de trein, de treinen]. This train stops at all stations en route.
– Sneltrein: fast/through train, express [noun]. This train stops at stations in the bigger towns and cities.
– Intercity: intercity train [noun] [de intercity, de intercity's]. This trains only stops at the stations in the "big" cities in the Netherlands. When in doubt what constitutes a bigger town or city in the Netherlands, it's best to check the (yellow) timetables that show which train stops where.