How to get to Hyvinkää/Riihimäki from the Airport
You can take a train or taxi to travel from the Helsinki Airport to Hyvinkää/Riihimäki. The easiest option is to take a taxi from the airport. It costs about €90 to Hyvinkää and about €100 to Riihimäki for 1-2 passengers. The cost of 1-2 additional passengers is around €10. Negotiate the price before taking the taxi.
I and P trains serve all stations along the line (Ring Rail line). Take train P from the airport to Tikkurila. The trains run every ten minutes Monday to Friday and Saturdays, and every 15 minutes on Sundays. The entrance to the train station at the Airport is in the corridor in close proximity to the arrival hall. Tickets can be bought only from the vending machines at the railway station. Buy the ticket right away till the end station of your journey. You can also buy your tickets online.
Change a train in Tikkurila. Take train R to Hyvinkää (Riihimäki direction, Hyvinkää is the previous stop before the end station Riihimäki). The trains run every 30 minutes. It takes 31 minutes to get to Hyvinkää and 39 min to Riihimäki. The train timetables can be found here https://www.hsl.fi/en.
You can take a look at the video of arrival instructions made by students.