In a displacement reaction of building a railway, how is the heat supplied?

N a single-displacement reaction, one element displaces another element in a compound.
The general equation for a single-displacement reaction is:
A + B-C → A-C + B
A and B must be either different metals (including H) or different halogens.
Chemists have devised an Activity Series. It lists the metals and nonmetals in decreasing order of chemical activity. An element that is higher in the series will displace an element that is below it. This enables chemists to predict which combinations will undergo single displacement reactions.

The formula is (nh4)2co3