@Simon You can join to the TransactionLine table and filter on "MainLine." Something like this:

SELECT
	Transaction.TranID,
	Transaction.TranDate,
	TransactionLine.Location,
	BUILTIN.DF( TransactionLine.Location ) AS LocationName
FROM
	Transaction
	INNER JOIN TransactionLine ON
		( TransactionLine.Transaction = Transaction.ID )
		AND ( TransactionLine.MainLine = 'T' )
WHERE
	( Transaction.Type = 'SalesOrd' )