Reposting the text of a comment posted by our admin Joydeep*^. You may find it useful.
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I study " Trains AT a Glance "
train number starting form those numbers belongs to
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101 > konkan railway
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110 > Mumbai CST & pune div.
112 > Jhansi div.
114 > Jabalpur , Nagpur , Solapur & bhusal div.
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120 > Satabdi & jansatabdi
121 > OLD CR ( now CR & WCR , NCR > jhansi div. )
122 > old NR
123 > Old ER ( ER , ECR - danapur , dhanbad , MGS div.)
124 > old NR ( now NR , NCR > ALD div , NWR > bikaner & jodhpur div.)
125 > NFR & old NER ( NER & ECR > sonepur & samastipur div )
126 > old SR ( SR & SWR > Bengalore & mysore div.)
127 > old SCR ( SCR & SWR > hubli div.)
128 > old SER ( now SER , ECoR & SECR )
129 > old WR ( now WR , NWR > Jaipur & ajmer div. )
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130 > Howrah div.
131 > Sealdah div.
132 > Danapur div.
133 > Dhanbad div.
134 > Malda town div.
135 > Assansol div.
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140 > Delhi div.
141 > Allahdabad div.
142 > Lucknow NR div.
143 > Moradabad div.
145 > Ambala div.
146 > Ferozpur div.
147 > Bikaner div.
148 > Jodhpur div.
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150 > Lucknow NER div.
151 > Varanasi NER div.
152 > Sonepur & samastipur div.
153 > Izatnagar div.
154 > alipurduar jn div
155 > Samastipur div. ( MG )
156 > Lumding div.
157 > katihar div.
158 > Rangiya div.
159 > Tinsukia div.
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160 > Chennai div.
161 > chennai div. ( MG , now form chennai Egmore )
162 > Mysore div.
163 > Thiruvananthapuram div.
165 > Bengalore div.
166 > Palkad div.
167 > madurai div.
168 > Triuchchirappalli div.
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170 > Secunderabad & hyderabad div.
172 > Vijayawada guntur div.
173 > Hubli div.
174 > Guntakal div.
176 > Nanded div.
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180 > kharagpur div.
181 > Chakradharpur div.
182 > Raipur & bilaspur div.
183 > Sambalpur div.
184 > Khurda road div.
185 > waltiar ( visakhapatnam ) div.
186 > Ranchi div.
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190 > Mumbai central div.
191 > Ahmedabad div.
192 > Rajkot Vadodara div.
193 > Ratlam div.
195 >
196 > Ajmer div.
197 > Jaipur div.
199 >
199 >