A carriage body for a multiple unit train, comprising: a chassis and a power system module mounted in a suspended manner below the chassis; wherein the chassis is provided with an access opening for facilitating to overhaul the power system module, and a floorboard of a passenger compartment covers a top surface of the chassis and surrounds the access opening, a door base is fixedly arranged on the chassis and around the access opening, a top surface of the door base is provided with a sliding groove, a nut-fixing block slides within the sliding groove relative to a length direction of the sliding groove, the sliding groove includes a first groove section with a width of its top opening smaller than a width of the nut-fixing block and a width of its middle part larger than the width of the nut-fixing block, and a second groove section with a width of its top opening larger than the width of the nut-fixing block and being communicated with the first groove section, wherein the nut-fixing block is placed into the sliding groove via the second groove section, and pushed to the first groove section from the second groove section, and the carriage body further comprises an access door for blocking the access opening, wherein the access door comprises a top panel, a reinforcement panel and a sound-insulation panel for sound-proofing, provided successively from top to bottom, the top panel, the reinforcement panel and the sound-insulation panel are fixedly connected as one piece; the periphery of the top panel is engaged with the floorboard of the passenger compartment; and the top panel is bolted to the nut-fixing block sliding into the first groove section of the door base via at least two bolts or screws. a chassis and a power system module mounted in a suspended manner below the chassis; wherein the chassis is provided with an access opening for facilitating to overhaul the power system module, and a floorboard of a passenger compartment covers a top surface of the chassis and surrounds the access opening, a door base is fixedly arranged on the chassis and around the access opening, a top surface of the door base is provided with a sliding groove, a nut-fixing block slides within the sliding groove relative to a length direction of the sliding groove, the sliding groove includes a first groove section with a width of its top opening smaller than a width of the nut-fixing block and a width of its middle part larger than the width of the nut-fixing block, and a second groove section with a width of its top opening larger than the width of the nut-fixing block and being communicated with the first groove section, wherein the nut-fixing block is placed into the sliding groove via the second groove section, and pushed to the first groove section from the second groove section, and the carriage body further comprises an access door for blocking the access opening, wherein the access door comprises a top panel, a reinforcement panel and a sound-insulation panel for sound-proofing, provided successively from top to bottom, the top panel, the reinforcement panel and the sound-insulation panel are fixedly connected as one piece; the periphery of the top panel is engaged with the floorboard of the passenger compartment; and the top panel is bolted to the nut-fixing block sliding into the first groove section of the door base via at least two bolts or screws.