You are looking at 10 x Brio Ikea Bigjigs Lillabo track connectors for train tracks
The beauty of these being 3d printed is they are cheaper than replacements from the shop and also you can pick any colour you want.
Please note: As these have changed over the years please ensure they are the same side holes and connectors as your existing ones - measurements are 35mm L x 12mm W x 12mm Tall and 6mm across the dog bone section