Station Codes are important codified formats for places in the transport industry, especially so in railways or civil aviation. Though both follow entirely different patters in India, certain 3-letter-codes are common to IR stations and Indian airports, but represent different cities, except for Gwalior which surprisingly has a common code for its railway station and its airport, as the following compilation reveals..!
Three-Letter-Code { Railway Station / Zone } (CivilAirport's IATA Code) [State/UT RTA]

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AGR Amargarh WR [MP] Agra [UP]
2 AGX Agastiyampalli SR[TN] Agatti [LD]
3 AJL Ajitwal NR[PB] Aizawl [MZ]
4 AMD Amila NWR [UP] Ahmedabad [GJ]
5 BBI Bara Bani ER[WB] Bhubaneswar [OR]
6 BDQ Budara ECR[JH] Baroda [GJ]
7 BEK Bundki NR[UP] Puttaparthi [AP]
8 BEP Bheerpur NCR [UP] Bellary [KA]
9 BHJ Parhaj Bazar NER[UP] Bhuj [GJ]
10 BHU Bhandu Motidau WR [GJ] Bhavnager [GJ]
11 BLR Balganur SWR [KA] Bangalore [KA]
12 BOL Bomadra NWR [RJ] Bombay [MH]
13 CBD Changrabandha NFR[WB] Car Nicobar [AN]
14 CDP Chandpara ER[WB] Cuddappah [AP]
15 DAE Dahegaon CR[MH] Daparizo [AR]
16 DEL Dendukulru SCR[AP] Delhi [DL]
17 DHM Dadhal Inam WR[GJ] Dharamsala [HP]
18 DIB Dibai NR[UP] Dibrugarh [AS]
19 DIU Dhing NFR[AS] Diu [DD]
20 DMU Dhamua ER[WB] Dimapur [NL]
21 GAU Gaur NER [UP] Guwahati [AS]
22 GOI Gevrai SCR[MH] Goa [GA]
23 GWL Gwalior NCR[MP] Gwalior [MP]
24 IDR Indi Road SWR [KA] Indore [MP]
25 JDH Jam Jodhpur Jn. WR[GJ] Jodhpur [RJ]
26 JGA Jogiara ECR[BR] Uamnager [GJ]
27 JLR Jetalsar Jn. WR[GJ] Jabalpur [MP]
28 JSA Jasai NWR [RJ] Jaisalmer [RJ]
29 KLH Khemli NWR [RJ] Kolhapur [MH]
30 KNU Kanjh WR[GJ] Kanpur [UP]
31 KTU Kuttipuram SR[KL] Kota [RJ]
32 KUU Kuhuri ECoR [OR] Kulu [HP]
33 LKO Lucknow Charbagh NR [UP] Lucknow [UP]
34 LDA Lidhora Khurd WCR [MP] Malda [WB]
35 NAG Nagargali SWR [KA] Nagpur [MH]
36 PAB Pattabiram SR [TN] Bilaspur [CH]
37 PBD Peddabrahmadevam SCR [AP] Porbunder [GJ]
38 PNQ Thrissur Punkunnam SR [KL] Pune [MH]
39 PNY Pandi SR [TN] Puducherry [PY]
40 RAJ Rajpipla WR [GJ] Rajkot [GJ]
41 RJA Rajanager ECR [BR] Rajamundry [AP]
42 RPR Raghunathpur ECR [BR] Raipur [CH]
43 RUP Rupahigaon NFR [AS] Rupsi [AS]
44 SLV Sandalpur NCR [MP] Shimla [HP]
45 STV Satyavada SCR [AP] Surat [GJ]
46 SXR Serndanur SR [TN] Srinager [JK]
47 SXV Suku ECoR [OR] Salem [TN]
48 TCR Thrissur SR[KL] Tuticorin [TN]
49 TIR Tirur SR [KL] Tirupati [AP]
50 TNI Tandarai SR [TN] Satna [MP]
51 TRV Tarva WR [GJ] Trivandrum [KL]
52 UDR Amdara WCR [MP] Udaipur [RJ]
53 VGA Vepagunda SR [AP] Vijayawada [AP]