There isn't one very nearby (1.1 mile away). If you like Mexican food checkout Olvera Street which is right across the way (a short walk) from Union station. Great food to be had there. Not really any "fast food" inside of Union station. Here's a link to the restaurants that are in Union station. https://goo.gl/3UnkNq Cheers!
There is no Jack in the Box inside the station. There is a Subway Sandwich shop and a mini mart called Trimana. I think there is also a place with salads and wraps.
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