The Railway Pioneer Regiment was a South African Raised unit.
His Record of Service is available free at http://angloboerwar.com/unit-information/south-african-units...
It shows he had 3 shillings a day stopped from his pay and sent to his Wife at 9 Main St. Frizington, Cumberland.
He was discharged in Johannesburg 31st March 1902 and gave his contact address as P.O. Frizington, Cumberland.
I have checked the QSA Medal Roll for his entry but there are no notes to say where his medal were sent to so it would appear it was issued to him before discharge.
Looks like Joseph married Elizabeth Mary Cowling in 1893
GRO Registration reference
Morton Joseph, Mar Qtr 1893, Registration District Whitehaven, vol 10b page 798
Cowling, Elizabeth Mary, Mar Qtr 1893, Registration District Whitehaven, vol 10b page 798
1901 Census his Wife is living at 8 Main Street, Frizington.
She did not claim to be head of family
MORTON, M E, Wife, Married, F, 31, where born Tavastock Doncaster, Yorkshire
MORTON, M A, Daughter, Single, F, 7, where born not given
MORTON, J W, Son, M, 3, where born not given
Address: 8, Main Street, Arlecdon, County: Cumberland
Sub District: Harrington, Enumeration District: 9, Ecclesiastical Parish: Frizington St Paul
Municipal Borough: Frizington
RG13 Pce 4889 Fo 156 page 1
Morton Mary Ann, Mar Qtr 1894, District Whitehaven, Vol 10b page 602
Morton John William, Sept Qtr 1897, District Whitehaven, Vol 10b page 668
In 1911 the family appear to be living at Skelsceugh Road, Frizington and Elizabeth (Lizzie) is recorded as a Widow.
Address: Skelsceugh Road Frizington Cumberland
MORTON, Lizzie, Head, Widow, age 40, Washer Woman, birth place England Cumberland
MORTON, Martha Ann, Daughter, Single, age 17, birth place Frizington Cumberland
MORTON, John William, Son, Single, age 1, birth place Frizington Cumberland
WHITTEN, Ellen Sister, Married, age 26, birth place England Cumberland
WHITTEN, Edna Mary, Niece, Single, age 4, birth place Arlecdon Cumberland
RG14, Piece: 31482.
Registration District: Whitehaven; Sub District: Harrington
Enumeration District: 8; Parish: Arlecdon
He had over 500 miles to cover from Johannesburg to the nearest main port of Durban, that is provided he chose to travel that route.
There are a number Morton deaths at sea between 1902 & 1911 but none have a given name. If he died in South Africa his death should have been registered in South Africa in which case a record should be in the SA Archives, but I cannot help with that detail.
You could check incoming UK passenger lists on Ancestry just in case he made it to the UK but not back home, or have a look for a death notice in the local newspapers held at the Cumbria Archive and Local Studies Centre, Scotch Street, Whitehaven.