Certainly! Here's a test case generator for the Reverse Polish Notation (RPN) problem:

```python
import random

def generate_rpn_expression():
    operations = ['+', '-', '*', '/']
    stack = []
    result = []
    num_operations = random.randint(1, 20)
    
    # Ensure valid RPN by alternating between numbers and operations
    for _ in range(num_operations):
        if len(stack) < 2:
            number = random.randint(-100, 100)
            stack.append(number)
            result.append(str(number))
        else:
            if random.random() < 0.5:
                # Add a number
                number = random.randint(-100, 100)
                stack.append(number)
                result.append(str(number))
            else:
                # Add an operation
                operation = random.choice(operations)
                result.append(operation)
                # Pop two numbers from the stack and push the result
                num2 = stack.pop()
                num1 = stack.pop()
                if operation == '+':
                    stack.append(num1 + num2)
                elif operation == '-':
                    stack.append(num1 - num2)
                elif operation == '*':
                    stack.append(num1 * num2)
                elif operation == '/':
                    # Ensure no division by zero
                    if num2 == 0:
                        num2 = 1
                    stack.append(num1 / num2)
    
    # Ensure the result RPN expression is valid by adding any remaining operations
    while len(stack) > 1:
        operation = random.choice(operations)
        result.append(operation)
        num2 = stack.pop()
        num1 = stack.pop()
        if operation == '+':
            stack.append(num1 + num2)
        elif operation == '-':
            stack.append(num1 - num2)
        elif operation == '*':
            stack.append(num1 * num2)
        elif operation == '/':
            if num2 == 0:
                num2 = 1
            stack.append(num1 / num2)
    
    return ' '.join(result)

def tcgen():
    num_datasets = random.randint(1, 50)
    test_cases = [generate_rpn_expression() for _ in range(num_datasets)]
    return test_cases

# Example usage
for test_case in tcgen():
    print(test_case)
```

### Explanation:
1. **generate_rpn_expression**: This function generates a single valid RPN expression:
   - It uses a stack to help ensure the expression remains valid.
   - It alternates between adding numbers and operations, ensuring at least two numbers are available for operations.
   - It handles edge cases such as ensuring no division by zero.
   
2. **tcgen**: This function generates the specified number of test cases (1 to 50):
   - Each test case is generated by calling `generate_rpn_expression`.

### Example Output:
```
10 2 12 - /
3 4 - 7 2 3 * + *
-1 -2 3 + +
```

This output mimics the format of the sample input provided in your problem statement. Each line is a valid RPN expression that can be evaluated.