"""Interprets a string operand as an appropriate type.

This function attempts to convert a string operand into an integer or float if it represents a valid number. If the string does not represent a valid number, it is returned as is. The function handles both integer and floating-point representations.

Args:
    operand (str): The string operand to interpret. If the operand is not a string, it is returned unchanged.

Returns:
    Union[int, float, str]: The interpreted operand as an integer, float, or the original string if no conversion is possible.

Examples:
    >>> preprocess_operands('123')
    123
    >>> preprocess_operands('123.456')
    123.456
    >>> preprocess_operands('abc')
    'abc'
    >>> preprocess_operands('123abc')
    '123abc'
"""
You should write code starting with:
from typing import Union

def preprocess_operands(operand: str) -> Union[int, float, str]:
    # TODO: Implement this function based on the docstring.
    pass

