20 C.F.R. § 220.141 : Employees' Benefits — RAILROAD RETIREMENT BOARD — REGULATIONS UNDER THE RAILROAD RETIREMENT ACT — DETERMINING DISABILITY — SUBSTANTIAL GAINFUL ACTIVITY — Substantial gainful activity, defined

Substantial gainful activity is work activity that is both substantial and gainful.

(a) Substantial work activity. Substantial work activity is work activity that involves doing significant physical or mental activities. The claimant's work may be substantial even if it is done on a part-time basis or if the claimant does less, gets paid less, or has less responsibility than when the claimant worked before.
(b) Gainful work activity. Gainful work activity is work activity that the claimant does for pay or profit. Work activity is gainful if it is the kind of work usually done for pay or profit, whether or not a profit is realized.
(c) Some other activities. Generally, the Board does not consider activities like taking care of one's self, household tasks, hobbies, therapy, school attendance, club activities, or social programs to be substantial gainful activity.

20 C.F.R. § 220.141