This is available and invoked after the last train has been used. The last train shows the strength of the hand and can signal to partner an interest in slam or shows better than a bottom hand
The problem with last train as it doesn't show control of that suit
To find out about control in the last suit you use Lackwood which is the second suit above the expected suit
If hearts are trump 4NT is Lackwood (remember 4♦ was describing hand strength not controls)
5♣ - 0 or 3 keycards for hearts, promises a diamond control
5♦ - 1 or 4 keycards for hearts, promises a diamond control
5♥ - No diamond control says nothing about keycards, if you didn't care about diamonds you should have used Kickback RKC
5♠ - 2 keycards for hearts without the Queen promises diamond control
5NT - 2 keycards for hearts with the Queen promises a diamond control
If spades are trump 5♣ is Lackwood (remember 4♥ was describing hand strength not controls)
5♦ - 0 or 3 keycards for spades, promises a heart control
5♥ - 1 or 4 keycards for spades, promises a heart control
5♠ - No diamond control says nothing about keycards, if you didn't care about diamonds you should have used Kickback RKC
5NT - 2 keycards for spades without the Queen promises a diamond control
6♣ - 2 keycards for spades with the Queen promises a diamond control
If last train hasn't been invoked then a jump above Kickback RKC is exclusion blackwood