openapi: 3.1.0
info:
  title: CTA Train Tracker API
  description: >-
    The Chicago Transit Authority Train Tracker API provides real-time
    train arrival predictions and run/location information for the CTA
    L train system. Access requires an API key issued by the CTA
    Developer Center.
  version: 1.0.0
  contact:
    name: CTA Developer Center
    url: https://www.transitchicago.com/developers/
  license:
    name: CTA Developer License Agreement
    url: https://www.transitchicago.com/developers/terms/
servers:
  - url: http://lapi.transitchicago.com/api/1.0
    description: CTA Train Tracker production
security:
  - apiKey: []
paths:
  /ttarrivals.aspx:
    get:
      operationId: getArrivals
      summary: Arrivals
      description: >-
        Returns train arrival predictions for a station (mapid) or
        platform (stpid).
      tags:
        - Arrivals
      parameters:
        - name: key
          in: query
          required: true
          description: CTA Train Tracker API key.
          schema:
            type: string
        - name: mapid
          in: query
          description: Numeric station identifier.
          schema:
            type: string
        - name: stpid
          in: query
          description: Numeric platform stop identifier.
          schema:
            type: string
        - name: max
          in: query
          description: Maximum results to return.
          schema:
            type: integer
        - name: rt
          in: query
          description: Train route code (e.g., Red, Blue, G, Brn, P, Pink, Org, Y).
          schema:
            type: string
        - name: outputType
          in: query
          description: Response format. Defaults to XML.
          schema:
            type: string
            enum: [xml, JSON]
      responses:
        '200':
          description: Arrival predictions
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ArrivalsResponse'
  /ttfollow.aspx:
    get:
      operationId: followTrain
      summary: Follow This Train
      description: Returns predictions for an individual train run.
      tags:
        - Follow
      parameters:
        - name: key
          in: query
          required: true
          schema:
            type: string
        - name: runnumber
          in: query
          required: true
          description: Train run number to follow.
          schema:
            type: string
        - name: outputType
          in: query
          schema:
            type: string
      responses:
        '200':
          description: Per-run prediction list
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ArrivalsResponse'
  /ttpositions.aspx:
    get:
      operationId: getPositions
      summary: Locations
      description: Returns the current latitude/longitude for in-service trains.
      tags:
        - Locations
      parameters:
        - name: key
          in: query
          required: true
          schema:
            type: string
        - name: rt
          in: query
          description: Comma-separated list of route codes.
          required: true
          schema:
            type: string
        - name: outputType
          in: query
          schema:
            type: string
      responses:
        '200':
          description: Positions of in-service trains
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/PositionsResponse'
tags:
  - name: Arrivals
    description: Train arrival prediction operations
  - name: Follow
    description: Follow an individual train run
  - name: Locations
    description: Train geolocation operations
components:
  securitySchemes:
    apiKey:
      type: apiKey
      in: query
      name: key
  schemas:
    Arrival:
      type: object
      properties:
        staId:
          type: string
        stpId:
          type: string
        staNm:
          type: string
        stpDe:
          type: string
        rn:
          type: string
        rt:
          type: string
        destSt:
          type: string
        destNm:
          type: string
        prdt:
          type: string
        arrT:
          type: string
        isApp:
          type: string
        isSch:
          type: string
        isDly:
          type: string
        lat:
          type: string
        lon:
          type: string
        heading:
          type: string
    ArrivalsResponse:
      type: object
      properties:
        ctatt:
          type: object
          properties:
            tmst:
              type: string
            errCd:
              type: string
            errNm:
              type: string
            eta:
              type: array
              items:
                $ref: '#/components/schemas/Arrival'
    Position:
      type: object
      properties:
        rn:
          type: string
        destSt:
          type: string
        destNm:
          type: string
        trDr:
          type: string
        nextStaId:
          type: string
        nextStaNm:
          type: string
        prdt:
          type: string
        arrT:
          type: string
        isApp:
          type: string
        isDly:
          type: string
        lat:
          type: string
        lon:
          type: string
        heading:
          type: string
    PositionsResponse:
      type: object
      properties:
        ctatt:
          type: object
          properties:
            tmst:
              type: string
            errCd:
              type: string
            errNm:
              type: string
            route:
              type: array
              items:
                type: object
                properties:
                  '@name':
                    type: string
                  train:
                    type: array
                    items:
                      $ref: '#/components/schemas/Position'