I don't have much military knowledge but Dave T on Sheffield indexes has been trying to help me out with this and here's what he posted on the forum if this helps anyone...
Combining information from Medal Cards, CWGC, Military Papers (FindMyPast) and the 1911 census we have found detailed info on two wen who signed up for the RFA in November/December 1910
RFA 63464 (just 6 numbers before your Thomas) was Sidney George Sains who died in WW1. His medal card shows he arrived in France on the same day as Thomas (16.8.14). From his number we can say he signed up within 5 days before Thomas, and most probably in the same place. Sidney was born in Burnham on Crouch, Essex and in the 1911 census was an RFA Gunner in Devon, so he had been posted quite quickly after signing up.
RFA 63502 was Thomas Robert Andrews and importantly his military papers survived. He signed up in LONDON on 29 Dec 1910 (so 28 days and 32 numbers later than your Thomas) and was posted only 2 days later as a Driver to the military establishment at Seaforth (Merseyside).He was born in Fulham and was discharged on 28th March 1911.He can be found at his home in Fulham on the 1911 census.
From this we can interpolate that Thomas McDonald signed up in London on
1.12.10 and was probably very quickly posted somewhere (Hillsborough?),