Musk posted a photo on Twitter early Thursday morning of a Hyperloop pusher pod he tested, saying in the caption that the pod reached up to 220 miles per hour.

“We took the SpaceX/Tesla Hyperloop pusher pod for a spin by itself a few days ago to see what it could do when not pushing student pods (some need a push to get going, e.g. passive maglev). Got up to 355 km/h (220 mph) before things started 🔥. Kind of like racing with a tugboat. Maybe able get past 500 km/h (about half speed of sound) next month with a few tweaks or maybe tiny pieces…” Musk posted.

Musk has been working on a Hyperloop that would take people from New York City to Washington, D.C., at amazingly fast speeds. Earlier in August, the company announced that it finished its second phase of testing, where a test pod reached 192 miles per hour.