Delft to Leiden

By train.

Would it be an intercity train? Thanks.

Perhaps, there are probably both intercities and other trains. Why are you asking?

Just go to the NS website: https://www.ns.nl/en/journeyplanner/#/

When I tried to book a train on thetrainline.com, it said there were no trains.

There are IC trains at 10 and 20 minute intervals all day long. Use the official NL train site given above.

Thank you, this is very helpful.

Avoid The trainline and other 3rd party sites, use www.ns.nl, the official website. Or, since this is the Netherlands, just buy the tickets in a machine when you arrive at the station.

The Trainline is nothing but an overcharging and incorrect info providing shady 3rd party reseller that should be avoided like the plague that they are. There, I said it.

There is absolutely no reason to put a middle man between you and the Dutch national train company when you buy tickets. The Dutch national railway company has an English website and accepts foreign credit cards.
In the Netherlands trains are mass transit. Tickets costs the same no matter if you buy them in advance or on the day of travel. There is free seating in trains and your tickets are not tied to a specific train. This means that a ticket from Delft to Leiden bought for today for instance, is valid on all trains from Delft to Leiden today. Check the schedule and buy your tickets at www.ns.nl/en or in their app. If you buy your ticket at the train station, you pay €1 extra to cover the cost of the disposable chip card.