Complex sentence: The assumption was that shares would inevitably rise in value once the UERL's tube railways were operational and producing a profit, giving investors in the notes the double benefit of the growth in share price and interest.
Split into simpler sentences: The assumption was that shares would inevitably rise in value once the UERL's tube railways were operational and producing a profit. Investors in the notes would gain the double benefit of the growth in share price and interest.