def split_rails(plaintext: str) -> str:
    """Split plaintext into 3 rails for encryption.

    Split the rails where the top rail is every 4th letter, the middle rail
    is every other letter starting at 1, and the bottom rail is every 4th
    letter starting at 2. After splitting, concatenate each rail and
    return the result.

    Args:
        plaintext (str): Plain text message without spaces or punctuation.

    Returns:
        String with message encrypted using 3 rail fence cipher.

    """
    top = plaintext[::4]
    middle = plaintext[1::2]
    bottom = plaintext[2::4]
    return top + middle + bottom