Interesting, why would you not bid last train? I wonder. Don't we need partner to have too many 'right' cards to make a slam? Assuming partner has a singleton in ♣, we need partner to hold ♦AKQ, ♥A and ♠A to make this a great slam. And if partner did hold all these cards, he hopefully will make a move over 4♥. Similarly, with a club void and relevant keycards, partner can make a move over 4♥. Obviously, there are hands like ♠AQxx ♥AJxx ♦AKJx ♣x where slam is good but even the absence of ♥J would reduce this to ~50% slam We need to pull 3 suits without a loser and I suspect this is against odds. Isn't this the whole point of 4♦ being last train? So we put a bit of pressure off partner and if partner has a minimum ish for his splinter then he can sign off.