The methods commonly used by Hong Kong railways to weld long rails are mainly Thermit welding and Flash Butt Welding. The hot press bonding method uses a special aluminum powder, mixed with iron oxide powder Fe2O3, and is welded by the principle of displacement reaction (ie, a more active metal to give a lower active metal from its compound). Extracted from it, the more active metal becomes a compound). The lightning-to-head welding method uses the resistance of the rail to heat the rail, allowing the rail interface to melt and engage. 1. Mix aluminum powder with iron oxide powder 2. Ignite it and produce iron solution Fe(l) when the heat reaches 1000 degrees Celsius 3. Inject the iron solution from the sand mold into the preheated rail joint as shown below: 4. After the rail is melted, it is joined 5. After the rail is cooled, remove the sand mold, and then smooth the rail surface, as shown below: Chemical formula: 2Al(s) + Fe2O3(s) ----> Al2O3(s) + 2Fe(l) Note: The alumina produced after the chemical reaction will float on the surface so it will not be mixed into the iron solution. The rail welded by the hot press crimping method has the advantage of only 85% of the strength of the base metal, but the advantage is that the equipment is simple and can be applied at the construction site TRANBBS , which is suitable for use in narrow places. Note: Both pictures are excerpted from the 1999 Gwangju Light Rail Services Commitment 1999 brochure. Welding procedure for lightning-to-head welding: 1. Clean the joint surface of the rail to be welded 2. Align the two rails and then apply a large current (requires low voltage) 3. Gently touch the two rails, and the high heat generated by the rail resistor will make the interface fused. 4. After the rail is cooled, smooth the surface The lightning-to-head welding method has a high welding strength, which is 99% of the base metal, but the required equipment is relatively large, so it needs to be built in the factory. Lightning head welding equipment From the factory to the construction site: Because the space on the rail site is generally small, the welding process will be carried out in the factory as much as possible. East Rail uses a thermo-compression method to weld each 25m long rail to a length of 100m in the factory, and then transport it to the site to connect with the rest of the rails in the same way; while the subway uses lightning-to-head welding method in the factory. Each 18m long rail is welded to a length of 72m and then transported to the site for thermal connection to the growth rail. Fire Alarm,Kidde Smoke Alarm,Fire Detector,Fire Alarm Beeping BEIJING FINECAST INTERNATIONAL CORPORATION , https://www.cfic-finecast.com
Welding procedure for thermal agent crimping: 
Welding method for rails of Hong Kong subway