This is primarily a demo app that uses the freely available CTA Bustracker API to allow users to see bus arrival predictions for specific routes and stops. It uses data provided by the CTA, and thus is as accurate (or inaccurate) as the provided data.
Key
string (required)
25-digit Bus Tracker API access key.

Rt
comma-delimited list of route designators (required if stpid not specified)
Alphanumeric designator of the route(s) (ex. “20” or “X20”) for which a list of service bulletins is to be returned. If combined with rtdir, only one route can be specified.
Rtdir
Single route direction (optional)
Direction of travel of the route specified in the rt parameter. The rt parameter is required when using the rtdir parameter.

Stpid
Comma-delimited list of stop IDs (required if rt not specified)

Set of one or more stop IDs for which service bulletins are to be returned. For example: 5029,1392,2019,4367 will return predictions for the four stops (if available). If combined with rt and rtdir, only one stop can be specified.
Base URL: http://www.ctabustracker.com/bustime/api/v1/getservicebulletins
Parameters:
Key string (required) 25-digit Bus Tracker API access key.  Rt comma-delimited list of route designators (required if stpid not specified) Alphanumeric designator of the route(s) (ex. “20” or “X20”) for which a list of service bulletins is to be returned. If combined with rtdir, only one route can be specified.
Rtdir Single route direction (optional) Direction of travel of the route specified in the rt parameter. The rt parameter is required when using the rtdir parameter.  Stpid Comma-delimited list of stop IDs (required if rt not specified)  Set of one or more stop IDs for which service bulletins are to be returned. For example: 5029,1392,2019,4367 will return predictions for the four stops (if available). If combined with rt and rtdir, only one stop can be specified.