In the city you will sometimes see those types of trucks with the original engine number in yellow as you saw in your pic, as have already probably figure out those are reserve trucks that are used to replace trucks that are in service in case of repairs or maintenance, they are also often used as temporary trucks while the original trucks might be participating in parades or special events. These are not to be confused with reserve trucks that are in service at the special stations of their perspective boroughs(sorry i dont remember the exact term for these stations, Similar to NYPD Support Services Borough, that have stations in each Borough to support local PCTS. ) and used to assist local stations however are rarely used and these stations usually use their other trucks such as rescue, collapse rescue, so on an so fourth. These trucks are number normally like those of the regular stations however the ones I have seen sometimes are in gold instead of white. Hope that helps.