UPDATED 2:30 p.m.: The gas leak near 7th/Northwestern has been resolved and the pipe repaired. Beginning with the 3:15 northbound departure, MetroRail service will be back to normal. Thank you for your patience.
Bus routes 4 and 100, which were detouring around the leak at 7th, are now back to regular route.
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9:45 a.m.: A gas leak near 7th/Northwestern has halted MetroRail service between MLK and Downtown stations this morning. There is no rail service south of MLK Station; however, buses will shuttle passengers between Downtown, Plaza Saltillo and MLK stations.
Bus routes #4 and 100 are also detoured around the leak at 7th.
The Austin Fire Department estimates it will take up to six hours to repair the pipe. This means the service interruption will continue most of the afternoon. Hopefully it will be repaired in time for the afternoon commute.
We will keep you updated throughout the day.
CapMetro Blog 
Bravo, the “shuttle bus” never arrived at Plaza Saltillo.
I’m sorry, let me look into it for you. Can you tell me what time you were waiting? Are you there now?
Nope, 9:16AM northbound train. I waited until around 9:30 before leaving. FYI, your LED signs said “seek alternate transportation” which if I hadn’t seen your FB post about shuttles, would’ve been perceived much differently. No cookies for me 😦