Array to string conversion on Insert

So i have a selector wich is based on my Database. Once people select something from that selector, select it and press next. They will see this page:

(Selected train02 in example here)

Code for this is:

<form method="POST" action="add_to_summary.php?user_id=<?php echo $_SESSION['user_id']?>">
            <div id="customer_list_table">
                <table>
                    <tr>
                        <th>Train id</th>
                        <th>Train Name</th>
                        <th>Tare Weight</th>
                        <th>Numbers of bogies</th>
                        <th>Numbers of axles</th>
                    </tr>
                    <?php 
                        foreach($_POST["checkbox"] as $key=>$val){
                        $data = $database->getAllAssoc_id($val);
                        foreach($data as $data1) { ?>
                            <tr>
                                <input type='hidden' name='user_id[<?php echo $_GET['user_id']?>]' value='<?php echo $_GET['user_id']?>'>
                                <input type='hidden' name='train_id[<?php echo $data1['train_id']?>]' value='<?php echo $data1['train_id']?>'>
                                <td><?= $data1['train_id'] ?></td>
                                <td><?= $data1['train_name'] ?></td>
                                <td><?= $data1['tare_weight'] ?></td>
                                <td><?= $data1['number_of_bogies'] ?></td>
                                <td><?= $data1['number_of_axles'] ?></td>
                            </tr><?php
                        }
                    } ?>
                </table>
                </div>
            <input name="Add to list" type="submit" id="add_to_list" value="Add to list">
        </form>

When they press Add to list, an insert gets send to my database like this:

function summary_add($id) {
            $sql = "INSERT INTO user_train_information "
        . "(train_id, user_id)"
        . "VALUES (:train_id, :user_id) ";
            $sth = $this->pdo->prepare($sql);
            $sth->bindParam(':train_id', $id, PDO::PARAM_STR);
            $sth->bindParam(':user_id', $id, PDO::PARAM_STR);
            $sth->execute();
    }

Code on the add_to_summary.php:

<?php
        foreach($_POST['user_id'] as $id) {
            $test = $database->summary_add($id);
        }
?>

Now, info gets inserted in my DB. but double the one i selected. like this:

And when i change $_POST['user_id'] to $_POST['train_id'] it inputs 14 2 times.

When i just do foreach($_POST as $id), i get 2 array to string conversion errors.

How do i fix this???

I want the table to insert correctly so that the user_id column will be 1, and the train_id column 14 EDIT:
for the selector / selection boxes: form pass selected to next page

EDIT:
It is working right now!

What is changed:

Changed the to hidden fields:

<input type='hidden' name='user_id' value='<?php echo $_GET['user_id']?>'>
<input type='hidden' name='train_id' value='<?php echo $data1['train_id']?>'>

The function:

function summary_add($train_id, $user_id) {
            $sql = "INSERT INTO user_train_information "
        . "(train_id, user_id)"
        . "VALUES (:train_id, :user_id) ";
            $sth = $this->pdo->prepare($sql);
            $sth->bindParam(':train_id', $train_id, PDO::PARAM_STR);
            $sth->bindParam(':user_id', $user_id, PDO::PARAM_STR);
            $sth->execute();
    }

Where i insert it:

<?php
            $test = $database->summary_add($_POST['train_id'], $_POST['user_id']);
            print_r($_POST);
?>